9.2 Expressing possibility / probability : The past

The perfect infinitive
Must / could / can't / might + have + past participle express degrees of probability in the past.

He must have been exhausted.
She can't have told him about us yet.
He might have got lost.
They could have moved house.

The continuous infinitive
Must / could / can't / might + have been + ing make the continuous forms in the past.

She must have been joking.
They can't have been trying very hard.
He could have been lying to you.

9.3 Asking about possibilities

To ask about possibility / probability we usually use Do you think ... ? Question forms with modal verbs of probability are unusual.

"Do you think she's married?" "She can't be."
"Where do you think he's from?" "He might be Spanish or Portuguese."
"Do you think they've arrived yet?" "They may have. Or they might have got stuck in the traffic."

9.4 So do I! Neither do I!

When we agree or disagree using So ... I / Neither ... I, we repeat the auxiliary verbs. If there is no auxiliary, use do / does /did. Be careful with sentence stress.

9.5 too and either / neither

We express that we have the same ideas as somebody else by using too and either / neither. With too and either we repeat the auxiliary verbs or, if there is not auxiliary, use do / does / did.

I like ice-cream.
I do, too. Me too.

I have always studied hard.
I have too. Me too.

I don't like working.
I don't, either. Me neither.

I can't play a musical instrument.
I can't, either. Me neither.