May, Might, Could, Must, and Can’t: Degrees of Possibility and Deduction

Modal verbs allow speakers to show how certain—or uncertain—they are about an explanation. This is a crucial part of natural conversation, professional judgement and careful reasoning.

May, might and could: open possibility

She may be in a meeting. She might be in a meeting. She could be in a meeting. All three can express possibility. In many contexts, the differences are small; might can sound slightly more tentative.

Must: strong logical deduction

Must can mean “the evidence strongly suggests this”: The office is dark and the door is locked. They must have gone home. This is deduction, not obligation.

Can’t: strong negative deduction

That can’t be the correct file; it has last year’s date. The speaker sees the possibility as logically incompatible with the evidence.

Could does not always mean weak certainty

Could may describe one possible explanation among several. Do not treat the modals as a rigid mathematical scale; context and intonation also affect strength.

Questions about possibility

English often uses Could it be…? or Do you think it might…? for open, diplomatic investigation.

Professional language

In meetings, replace unsupported certainty with evidence-based modality: The delay may be related to the supplier change. If the evidence is strong: The mismatch must be coming from the old dataset.

Practice

Choose the best modal: The lights are on and I can hear voices; they ___ be inside. / I am not sure where Dana is; she ___ be travelling. / This invoice says 2023; it ___ be the new one.

Suggested answers: must; may/might/could; can’t.