Read these sentences and choose the correct answer to fill in the gaps. 1. That's ridiculous- you be joking.

can
must

2. It be the postman; it's far too early.

mustn't
can't

3. I do my homework.

Correct answer 1:  must

Correct answer 2:  can't

Correct answer 3:  must

Объяснение:

Modal verb 'can'

Can is an auxiliary verb, a modal auxiliary verb. We use can to:

- talk about possibility and ability

- make requests

- ask for or give permission

The main verb is always the bare infinitive (infinitive without "to").

subject

auxiliary verb

main verb

+

I

can

play

tennis.

-

He

cannot/can't

play

tennis.

?

Can

you

play

tennis.

Modal verb 'must'

We often use must to say that something is essential or necessary, for example:

Must is a modal auxiliary verb. It is followed by a main verb.

subject

auxiliary verb

main verb

I

must

go

home.

You

must

visit

us.

We

must

stop

now.

In general, must expresses personal obligation. Must expresses what the speaker thinks is necessary. Must is subjective.

In each of the above cases, the "obligation" is the opinion or idea of the person speaking. In fact, it is not a real obligation. It is not imposed from outside.