English Modal Verbs

Modal verbs are the small words that let you express far more than plain facts, ability, possibility, permission, obligation and advice all run through them. In English the modal verbs are can, could, may, might, must, shall, should, will and would, together with the near-modal phrases have to, ought to and be able to. They behave differently from ordinary verbs, and once you know their shared rules you can use every one of them with confidence. This guide sets out what modal verbs are, what each group means, and how to form questions and negatives, with examples and a quick reference table.

A modal verb is an auxiliary, a helping verb, that sits before a main verb and changes its meaning, showing how likely, necessary, permitted or advisable something is. English modals share a set of features that set them apart from ordinary verbs: they are followed by the bare infinitive, the verb without "to", as in "she can swim", not "she can to swim"; they do not add "-s" in the third person singular, so it is "he must go", never "he musts go"; and they form questions and negatives on their own, without "do", as in "Can you help?" and "You should not worry". The phrases have to, ought to and be able to carry similar meanings but follow ordinary verb rules.

Ability: can, could and be able to

Use "can" for present ability and "could" for past ability: "I can drive", "I could read before I started school". Where "can" and "could" have no form, use "be able to", which works in every tense: "I will be able to help tomorrow", "she has been able to save money". For a single past achievement, use "was or were able to" or "managed to" rather than "could": "we were able to catch the train".

Permission and requests: can, could, may and might

"Can" and "could" ask for and give permission, with "could" the more polite: "Can I sit here?", "Could I ask a question?". "May" is more formal: "May I come in?". To make a request, "Could you…" and "Would you…" are softer and more courteous than "Can you…".

Obligation and necessity: must, have to and mustn't

"Must" and "have to" both express obligation. "Must" often reflects the speaker's own sense of duty, as in "I must call her back", while "have to" points to an outside rule, as in "passengers have to wear a seatbelt". The negatives differ sharply, and this is the point learners most often miss: "mustn't" means something is forbidden, as in "you mustn't smoke here", whereas "don't have to" means it is simply not necessary, as in "you don't have to come if you are busy".
Advice and expectation: should and ought to
"Should" and "ought to" give advice or state what is expected: "you should rest", "the parcel ought to arrive today". "Had better" is stronger and warns of a consequence: "you had better leave now, or you will miss it".

Possibility and probability: may, might, could and must

To say something is possible, use "may", "might" or "could": "it may rain later", "she might be at home", "that could be the answer". To draw a confident conclusion, use "must": "the lights are on, so they must be in".
Offers, promises and predictions: will, would and shall
"Will" makes predictions, offers and promises: "it will be fine", "I will carry that for you". "Would" is its softer, more hypothetical form, used for polite requests and imagined situations: "I would love to come", "would you mind waiting?". "Shall" is chiefly used in questions offering help or making suggestions: "Shall I open the window?".