Modal passives: obligation, permission, advice or possibility

Must be signed and has to be signed can express very similar obligations. Must often presents the speaker’s conviction or authority, or a formal written rule such as a sign or regulation. Have to often points to an external requirement independent of the speaker. This is a tendency, rather than an absolute distinction.

The two are interchangeable in many everyday contexts. Speakers often choose have to as a more neutral conversational form. The distinction is most noticeable in formal wording or when a speaker explicitly emphasises insistence.

In rules of conduct, mustn’t / must not be done and can’t / cannot be done can express prohibition. Can’t also expresses impossibility: This file can’t be opened. Distinguish these from absence of obligation: doesn’t have to be done.

Doesn’t have to be done / don’t have to be done and needn’t be done express absence of obligation: doing the action is optional. Confusing this with prohibition reverses the intended message.

Use should be done and ought to be done for advice or recommendations. They can also express moral obligation or expectation, so the grammar alone does not guarantee that ignoring the recommendation has no consequences. Must / have to expresses a requirement more directly.

Ought to is less common in modern conversation, especially in negatives and questions. Should is a much more usual default; ought to can sound slightly more formal or literary.

Can be done often describes ability or general possibility: something can technically be done. Can also expresses permission: someone is allowed to perform the action.

May be done is a more formal written permission pattern in rules, notices or contracts. Could be done often describes hypothetical ability or offers a tentative suggestion, while might for permission is rare and sounds very formal or old-fashioned.

May be done, might be done and could be done can express uncertain possibility: the speaker does not know whether the action will happen. Might can sound more tentative than may, but the difference is often slight and the forms can be interchangeable; context matters.

Will be done can express a prediction, intention or promise. It does not guarantee the real outcome and can occur with probably. May, might and could present possibility more tentatively; the strength of the whole statement also depends on context.

The mechanical structure—unchanged be after a modal, choosing a past participle and ordering negatives or questions—is covered by Modal + be + past participle in the same topic. Here the form is a tool; the target is the choice of meaning.

Modal + have been + past participle evaluates a completed past action: should have been done expresses what ought to have happened; must have been done expresses a deduction. Simple could be done can also describe past possibility. Have/get something done, meaning arranging for a service, is studied separately.