1: Talking about the present:
must / might / could / may / can’t + infinitive
I am waiting for Amanda with another friend, Danny.
I ask: Where is Amanda?
Notice that the opposite of ‘must‘ is ‘can’t in this case.
Will / won’t
We use will and won’t when we are very sure:
Should / shouldn’t
Should and shouldn’t are used to make an assumption about what is probably true, if everything is as we expect:
This use of should isn’t usually used for negative events. Instead, it’s a better idea to use will:
Can is used for something that is generally possible, something we know sometimes happens:
Can is not used to talk about specific possibilities:
2: Using modal verbs to talk about the past:
must / might / could / may / can’t + have + past participle
You: Where was Sandra last night?
Will / won’t + have + past participle
Will and won’t / will not + have + past participle are used for past certainty (compare with present use of ‘will’ above):
Should + have + past participle
Should + have + past participle can be used to make an assumption about something that has probably happened, if everything is as we expect (compare with present use of ‘should’ above):
We can use could + infinitive to talk about a general possibility in the past (compare with the use of ‘can’ above):
This is not used to talk about specific possibilities in the past (instead we use could + have + past participle):