Modal Verbs: Obligation and Permission in Technical English
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An API security policy states: "All API clients _____ authenticate using a valid Bearer token on every request." Which modal expresses a strict, non-negotiable requirement?
Must is correct. In technical specifications following RFC 2119, MUST indicates an absolute requirement. Non-compliance means the implementation is non-conformant. "May" grants optional permission. "Should" is a recommendation (SHOULD in RFC 2119 means there may be valid reasons to ignore it). "Need" is not a standard RFC modal — in specs, use "must" or "need to" for requirements.
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A REST API reference says: "Clients _____ pass an Idempotency-Key header to prevent duplicate requests, but it is strongly recommended for write operations." Which modal best signals an optional but strongly recommended behaviour?
Should is correct. RFC 2119 defines SHOULD as: "there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course." The phrase "strongly recommended" confirms this — the client is not obligated, but deviation requires justification. "Must" is absolute. "May" is merely optional. "Shall" has the same force as "must" but is more formal/legalistic.
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A compliance document states: "Developers _____ not store plaintext passwords in any database, log file, or environment variable." Which modal signals an absolute prohibition?
Must is correct. "Must not" (MUST NOT in RFC 2119) is an absolute prohibition — no exceptions. "Should not" is a strong recommendation against, but allows for justified exceptions. "May not" has ambiguity in everyday English (it can mean "is not allowed to" or "might not") — in technical specs, avoid using "may not" for prohibition; use "must not" instead. "Need not" means "is not required to" — which is the opposite of a prohibition.
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A runbook instruction reads: "Engineers on call _____ acknowledge an alert within 5 minutes of receipt during business hours." Which modal expresses an organisational obligation?
Are required to is correct in this compliance context. The runbook establishes a binding SLA-level obligation — not merely a recommendation. "Are required to" conveys the same absolute obligation as "must" but in a more formal administrative register. "Should" is a recommendation. "May" is permission. "Might" is speculative possibility, entirely inappropriate here.
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An API reference states: "The fields query parameter _____ be used to request a partial response containing only specified fields." Which modal grants optional permission without implying a recommendation?
May is correct. RFC 2119 defines MAY as optional — the user may choose to use this feature or not. It is neither recommended nor discouraged. "Must" implies obligation. "Should" implies a recommendation that most users should follow. "Shall" is equivalent to "must" — it indicates a requirement, not an option.
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A security policy states: "Service accounts _____ have more permissions than are strictly necessary for their function." Which modal creates a prohibition aligning with the principle of least privilege?
Must not is correct. The principle of least privilege is a security requirement, not merely a guideline — service accounts are prohibited from having excess permissions. "Must not" (= MUST NOT in RFC 2119) is the strongest prohibition. "Should not" indicates it is inadvisable but allows exceptions. "May not" is ambiguous (prohibition or possibility?). "Need not" means "is not required to" — the opposite meaning.
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A technical spec reads: "The client _____ retry the request using exponential backoff if the server returns a 503 response." Which modal best expresses a strong recommendation that allows exceptions?
Should is correct. Retry logic is a best practice recommendation — most clients should implement it, but an internal tool making one-off requests may legitimately omit it. "Should" (RFC 2119 SHOULD) signals this recommendation-with-flexibility. "Must" would make retry mandatory for all compliant clients. "May" would suggest it is merely permitted, not encouraged. "Will" is a future statement, not a requirement.
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Which sentence uses an obligation/permission modal INCORRECTLY in a technical context?
Option C is ambiguous and therefore incorrect for a formal spec. "May not" has two possible readings: (1) permission denial — "is not permitted to", or (2) possibility — "might not be." In a technical or legal specification, this ambiguity is unacceptable. Use "must not" for prohibition. The intended meaning is almost certainly prohibition, so the correct phrasing is: "The API must not be used for real-time financial trading without a dedicated licence." Options A, B, and D all use modals correctly.
9 / 22
A developer portal note says: "You _____ upgrade to API v3 immediately — v2 will be deprecated next quarter." Which modal most accurately expresses urgent advice without making it a strict requirement?
Should is correct. "You should upgrade immediately" conveys strong advice — the speaker believes it is the right course of action and that the reader will likely face negative consequences for ignoring it, but it is not technically mandatory. "Must" would imply that continued use of v2 is prohibited now, which contradicts "next quarter." "May" implies it is only an option with no recommendation. "Can" expresses capability, not advice.
10 / 22
An infrastructure policy document states: "Production deployments _____ only be triggered by the CI/CD pipeline — direct SSH access to production servers _____ not be used for deployments." Which pair of modals is correct?
Must / must is correct. Both clauses are absolute requirements — one affirmative obligation (deployments must go through CI/CD) and one absolute prohibition (manual SSH deployments are forbidden). This is a security policy, not a recommendation. "Should / can" makes both rules advisory. "May / need" introduces optional permission where mandatory control is intended. "Can / should" creates a capability statement and advisory, not enforceable policy.
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A new API endpoint requires a specific format for the request body. The documentation states: 'The input data must be in JSON format with the following schema...'. Which modal best expresses this requirement?
The word 'must' is a strong obligation. It signifies a non-negotiable requirement that the data *absolutely* conforms to the specified JSON schema. The other options denote weaker suggestions or permissions.
12 / 22
A new API endpoint requires a specific format for the request body. The documentation states: 'The input data must be in JSON format with the following schema...'. Which modal best expresses this requirement?
The word 'must' is a strong obligation. It signifies a non-negotiable requirement that the data *absolutely* conforms to the specified JSON schema. The other options denote weaker suggestions or permissions.
13 / 22
A new API endpoint requires a specific format for the request body. The documentation states: 'The input data must be in JSON format with the following schema...'. Which modal best expresses this requirement?
The word 'must' is a strong obligation. It signifies a non-negotiable requirement that the data *absolutely* conforms to the specified JSON schema. The other options denote weaker suggestions or permissions.
14 / 22
During a code review of a new microservice, Alex comments: 'The service should always validate user input against the schema before processing it.' Which modal verb best expresses this requirement in the context of code quality and security?
'Must' indicates a strong obligation or necessity. In this scenario, Alex is emphasizing a critical requirement for data integrity and preventing vulnerabilities. 'Should' suggests advice, while 'can' and 'may' indicate possibility rather than mandatory action. The use of 'must' clearly conveys the seriousness of the validation process.
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In a Slack channel discussing an upcoming deployment, Sarah writes: 'The team needs to thoroughly test the new version before releasing it to production.' Which modal best reflects her assessment of the urgency and importance of this task?
'Have to' denotes an obligation or necessity derived from external factors (e.g., a requirement). This aligns with Sarah's emphasis on the critical need for testing before deployment. 'Will' expresses prediction, 'could' suggests possibility, and 'may' indicates permission without obligation – neither accurately reflects her strong assertion of urgency.
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A developer is preparing a Pull Request description for a new feature. They state: 'Users are permitted to upload files up to 10MB in size.' Which modal best conveys the permission granted without implying a recommendation or restriction?
'Are permitted' explicitly grants access to a resource or action. This is crucial for clear communication regarding user capabilities. 'Must,' 'should,' and 'have to' all imply obligation, while 'can' suggests possibility, which isn't the intent here.
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During a standup meeting, David says: 'Engineers on call must immediately investigate any alerts indicating critical system failures.' Which modal best expresses the organizational obligation regarding incident response?
'Must' indicates a strong, mandatory requirement. In this context, it underscores the immediate action expected of engineers during critical incidents. 'Can,' 'will,' and 'may' all imply lesser levels of obligation or permission – 'must' is the most appropriate for conveying an urgent responsibility.
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Mark, a senior engineer, is reviewing a draft PR for a new authentication service. He needs to ensure the team adheres to security best practices. 'The API endpoint should always return a 200 OK status code if the request is valid.' Which modal verb does Mark MOST appropriately use to convey this requirement?
'Must' indicates a strong obligation and is suitable for critical security requirements. 'Should' expresses advice or recommendation, which isn't as forceful in this context. 'Can' and 'May' offer permission, lacking the necessary emphasis on enforcement. Therefore, 'must' clearly communicates the expected behavior.
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Liam, a DevOps engineer, is drafting an automated deployment script. He needs to specify that developers are allowed to override certain configuration settings during testing. 'Developers may temporarily adjust the database connection string for troubleshooting.' Which modal best reflects this allowance?
'Can' expresses permission and indicates that the action is permissible under specific circumstances. 'Will' and 'Shall' imply a future obligation which isn't relevant here. 'Must' denotes a requirement, contradicting the idea of an override being allowed.
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Chloe, a technical writer, is documenting a new API call. She needs to state that clients are required to include a specific header in their requests. 'The client is required to include the 'X-Request-ID' header.' Which modal verb best expresses this requirement?
'Must' indicates a strict requirement and is appropriate for mandatory API parameters. 'Should' suggests a recommendation rather than a necessity. 'May' implies permission, while 'Could' presents possibility which isn't fitting here.
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Daniel is writing a Slack message to the team about an upcoming system upgrade. He wants to ensure they take necessary precautions. 'Before initiating the upgrade, engineers should immediately back up their local databases.' Which modal verb best expresses this advice?
'Must' is ideal for conveying a strong recommendation that carries significant weight. It's appropriate when emphasizing the importance of following a protocol to avoid potential problems during an upgrade. 'Will' suggests future action but doesn't carry the same level of obligation as 'must'.
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Eva is creating a PR description for a bug fix. She needs to instruct developers on how to handle a specific error condition. 'Developers can retry the API call up to three times with a 5-second delay if it fails.' Which modal verb best expresses this guidance?
'May' indicates permission and acknowledges that developers have the discretion to attempt a retry under certain circumstances. 'Should' offers advice, while 'Must' denotes a requirement, and 'Will' suggests a future obligation which isn't appropriate in this scenario.
What will I practise in "Modal Verbs: Obligation and Permission in Technical English — Grammar Exercise"?
Practice must, have to, need to, should, may, and can in API documentation, security policies, and technical specifications to express requirements
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This module has 22 multiple-choice exercises, each with instant feedback and a full explanation of the correct answer.
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Every exercise is written by the CoderSlingo team, drawing on real workplace English used in IT roles, then reviewed for accuracy and clarity.