I.   Modals of deduction (certainty,possibility, impossibility)

1.   Must: certainty:
a.   Present: must + infinitive of the verb
He has a house in London and another in paris, so he must be rich
That man must be a doctor
b.   Past: must + have + past participle of the verb
She must have passed her driving test. I saw her driving her father’s car alone
He must have taken sleeping pills last night. He did not wake up till lunch time
NB : must is not used for negative deduction and is not normally used in the interrogative

2.   Can’t: impossibility (negative deduction):

a.   Present: can’t + infinitive of the verb
It can’t be a car. I t must be a lorry
b.   Past: can’t + have + past participle of the verb
A man stole your phone. It can’t have been my brother
NB : “can’t” is used in negative instead of “must”

3.   May/ might: possibility:

a.   Present: may/might + infinitive of the verb
He may/ might leave tomorrow (perhaps he will leave)
He may/ might marry next year (perhaps he will marry)
b.   Past: may/ might + have + past participle of the verb
He may/ might have left ( it is possible that he left)
He may/ might have gone (it is possible that he went)

4.   Can/ could: possibility:

a.   Present: can/ could + infinitive of the verb
It can/ could rain tomorrow
You can/ could leave now if you want
b.   Past: can/ could + have + past participle of the verb
They can/ could have brought the computer yesterday (they had the possibility to bring the computer, but they didn’t bring it)

    II.   Modals of obligation:
1.   Must/ have to: obligation
a.   Present: must/ have to + verb without“to”:
You must stop smoking
You have to pray
b.   Past: had to
You had to stop smoking
2.   Ought to/ should: obligation:
a.   Present: ought to/ should + infinitive of the verb
You ought to respect your parents
They should allow parking here
Students should be on time
NB: with “ought to/ should” we do not necessarily feel that the obligation is being or will be fulfilled
b.   Past: ought to/ should + have + past participle of the verb
You ought to have written the essay (you didn’t)
You oughtn’t to have shouted at your mother (you did)
You should have attended your friend’s party (youdidn’t)
You shouldn’t have smoked in the class (you did)
NB: “ought to/ should + have + pp of the verb” express an unfulfilled obligation. In the negative, they express a wrong or foolish action in the past.