Modals are verbs that modify other verbs. The most common modals are:
Can
Should
Must
Note that all subjects take the same form of the modal.
Positive
Subject + Modal + Base Form of Verb + Objects
Examples
He can play the piano.
I must leave soon.
Negative
Subject + Modal + Not + Base Form of Verb + Objects
Examples
They can't visit next week.
You shouldn't go to that film.
Question
Modal + Subject + Base Form of Verb + Objects
Examples
Can you help me?
What should I do?
Giving Advice with Should
'Should' is used when asking for or giving advice. It is also used when asking for suggestions.
Examples
I think you should see a doctor.
What type of job should I get?
Expressing Ability with Can
'Can' is used to speak abilities.
Examples
He can speak Japanese.
Can you play golf?
Asking for Permission with May
'May' is used to ask for permission.
Examples
May I help you?
May I visit you this afternoon?
NOTE: In spoken English, 'Can I ...?' is often used instead of 'May I ...?'
طرزشناختن زمان گذشته کامل (ماضی بعید)