Теорія:

Study the information about modal verbs.
  
The two uses of modal verbs.
 
1. Verbs like can and may are modal auxiliaries. We often refer to them as modal verbs.
There are ten modal verbs: can, could, may, might, will, would, shall, should must, ought to and three ‘semi-modals’: need, dare, used to.
 
In their first use, modal verbs have basic meanings which are given in dictionaries.
 
Modal verbs are not ‘complete verbs’. We use verbs like must and can to refer only to the present or the future.

The second way we use modals is to express degrees of certainty and uncertainty. We use all modals (except shall). We express the greatest uncertainty with might, the greatest uncertainty with must/can’t.
 
He must be right. He must know the answer.
She can’t be right. She can’t know the answer.
 
In their second use, modals have only two forms.

1. Present form. He must be right. He must know the answer.
2. Perfect and past form. He must have been right. He must have known the answer.
Джерела:
www.stanag6001.com
L.G. Alexander. Longman English Grammar practice for intermediate students- Harlow: Longman, 296p.