by Liz Walter
The main modal verbs in English are: can, could, may, might, must, ought, shall, will and would.
They are used to express important ideas:
Alice might/will come later. (degrees of probability)
You should call your mother. (advice)
Often the same modal verb can be used with different functions.
You must wear a helmet. (giving instructions)
His children must be grown up now. (expressing certainty)
Can you play the piano? (asking about ability)
Can we take photos? (asking for permission)
There are some important things to remember about modal verbs. Firstly, they do not inflect, by which I mean, they do not add -s, -ing, -ed etc. in the way that ordinary verbs do.
Secondly (apart from in short answers, etc.), modal verbs are always followed by another verb, and the second verb is always in the infinitive (base) form without ‘to’.
We could go for a walk.
We could to go for a walk.
We could going for a walk.
There is one exception to this rule, which is the modal verb ought, which does have ‘to’ before the second verb:
You ought to work harder.
The most common way to form negatives of modal verbs is to add ‘n’t’:
We couldn’t get the door open.
Lulu mightn’t have got your message.
In very formal writing only, it’s best to use the full word not for negatives. This is written as a separate word for all modal verbs except can, where the formal negative is the single word cannot:
In such cases, the contract shall not be considered valid.
Goods cannot be returned after 14 days.
Students often make errors with the modal verb must. For more information about when to use must and when to use have to, see my colleague Kate Woodford’s post from back in 2014.
For people with English as their mother tongue, the most frequent error is probably the use of ‘of’ rather than the correct ‘have’ when writing about something in the past:
You should have told her where we were.
You should of told her where we were.
They might have seen it.
They might of seen it.
The uses of modal verbs are quite subtle. You will learn basic ones (such as can to talk about things you are able to do) quite early in your studies. More complex ideas (such as must to express ideas you believe to be true but cannot prove beyond doubt) come later. However, if you follow the basic rules above, you will at least avoid most errors in their grammar.