What are modal verbs? How do you use them?

Modal verbs (can, could, may, might, will, shall, would, should and must) are followed by the 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.

Be careful when choosing between the modal verb 'should' and the word 'ought', because they have the same meaning but different grammar. Ought is followed by a to-infinitive:

• You ought to know that by now.

• They ought to have insisted on some compensation.