Zero

Certainty

Must

Present

When we feel sure that something is true we use must .

Example

This restaurant is always full of people.It must be good .(I am sure this restaurant is good )


Evidence
In this example we are sure that this restaurant is good because we base our certainty on the evidence that it is always full of people .

Past

In the past we use must have +past participle to express certainty .

Example

They bought a very expensive car .They must have been rich .( I am sure they were rich because they bought a very expensive car .

Negative deduction

Present

Can't

The opposite of must in this sense is can't .

When we feel sure that something is impossible we use can't .

Example

This restaurant is always empty.It can't be good .(I am sure it is not good because it is always empty ).

Past

In the past we use can't have +past participle to say that something was impossible .

Example

They can't have known many people because they lived only one month in this district .( I am sure they didn’t know many people because they didn’t live so long in this district )

Remember

Can't
Must

Can't have+past participle
Must have +past participle

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