The infinitive after modals

The infinitive is used after modal verbs, semi-modal verbs (also called marginal modals) and other modal expressions.

Modal verbs

Modal verbs (can, could, may, might, will, shall, would, should and must) are followed by a bare infinitive:

I could hear the dog barking outside.
You must be joking.
He may have caught the train.
You should have told me earlier.

Semi-modal verbs (marginal modals)

Used is followed by a to-infinitive:

They used to live by the sea.

Ought is followed by a to-infinitive:

You ought to know that by now.
They ought to have insisted on some compensation.

When used as modal verbs, dare and need are followed by a bare infinitive:

Need you make so much noise?
You needn't have been so rude.
How dare she criticise us?

When used as a main verb, need is followed by a to-infinitive:

Do you need to use the hairdryer?

Dare as a main verb can take a bare infinitive or a to-infinitive:

He didn't dare (to) look back.

But in the expression Don't you dare..., it always takes a bare infinitive:

Don't you dare interrupt me!

Other modal expressions

Other modal expressions are usually followed by a to-infinitive, with the exception of had better and would rather, which are followed by a bare infinitive:

Although the current was strong, I was able to swim to the other bank.
Will journalists be allowed to ask questions?
Hurry up! The ceremony is about to begin!
Kevin is stuck in a traffic jam, so he is bound to be late.
Negotiations were due to take place later that week.
Smokers are more likely to develop lung cancer compared to non-smokers.
You are not to do that again.

We had better leave soon.
I'd rather go by bike than walk.