A : My house is next to the freeway.
B : It must be very noisy.
We use must to say that we feel sure something is true:
We use must to say that we feel sure (that) something is true:
We use can to say that something is possible or allowed, or that somebody has the ability to do something.
「to do」 – 何かをするための能力
「to do (something)」 – 何かをするための能力
base form – 原型
You’ve been traveling all day. You must be tired.
(Traveling is tiring and you’ve been traveling all day, so you must be tired.)
“Jim is a hard worker.” “Jim? must be joking. He’s very lazy.”
I’m sure Sally gave me her phone number. I must have it somewhere.
We use must not to say that we feel sure something is not true:
Their car isn’t outside their house. They must not be home. (= They must be out)
Brian said he would be here by 9:30. It’s 10:00 now, and he’s never late. He must not be coming.
They haven’t lived here very long. They must not know many people.
Study the structure:
I / you / he (etc.) | must (not) | be (tired / hungry / home, etc.)
I / you / he (etc.) | must (not) | be (doing / coming / joking, etc.)
I / you / he (etc.) | must (not) | do / get / know / have, etc.
Must (not) have done
For the past, we use must (not) have done:
“We used to live close to the freeway.” “Did you? It must have been noisy.”
There’s nobody at home. They must have gone out.
I’ve lost one of my gloves. I must have dropped it somewhere.
She walked past me without speaking. She must not have seen me.
Tom walked into a wall. He must not have been looking where he was going.
Study the structure:
I / you / he (etc.) | must (not) | have | be (tired / hungry / home, etc.)
I / you / he (etc.) | must (not) | have | be (doing / coming / joking, etc.)
I / you / he (etc.) | must (not) | have | do / get / know / have, etc.
Can’t and must not
It can’t be true = I believe it is impossible:
How can you say such a thing? You can’t be serious!
Compare can’t and must not:
A : Joe wants something to eat.
B : But he just had hunch. He can’t be hungry already. (= it’s impossible that he is hungry because he just had hunch)
A : I offered Bill something to eat, but he didn’t want anything.
B : He must not be hungry. (= I’m sure he is not hungry – otherwise he would eat something)