We must be early. Nobody else has arrived yet.
They work at the same office so they may know each other.
She might not be in. The lights are all out.
That can't be Mark's car. He told me his was in the garage.

Different modal verbs tell us how sure about a deduction we are:

It's cold in that house.	Fact: I know* it is.*
It must* be cold in that house.*	Deduction: I'm sure* it is.*
It may / might/ could* be cold in that house.*	Deduction: It's possible* that it is.*
It may / might not* be cold in that house.*	Deduction: It's possible* that it isn't.*
It can't* be cold in that house.*	Deduction: I'm sure* it isn't.*
It isn't cold in that house.	Fact: I know it isn't.

The opposite of must for deductions is can't. Don't use mustn't, can or couldn't for deductions:

This bill can't be right. I only ordered a salad.
NOT ~~This bill couldn't / mustn't be right.~~

There must be a mistake.
NOT ~~There can be a mistake.~~

There is no difference between may, might and could. All three mean that something is possible.
To make deductions about actions happening now, use a modal + be + verb + -ing:

She isn't answering the phone. She might be listening to music.

Vocabulary

Houses and buildings

accommodation	a place where you live or stay
attic	a room at the top of a house under the roof
balcony	a small area joined to the wall outside a room on a high level where you can stand or sit
basement	a room or set of rooms that is below ground level in a building
block of flats	a large building that is divided into apartments
cottage	a small house in the countryside
doorbell	a button that you press next to a door that makes a noise to let someone know that you are there
first floor	the level of a building directly above the ground level
flat	a set of rooms to live in, with all the rooms on one level of a building
front door	the main entrance to a building, especially a house, usually facing the road
ground floor	the level of a building that is on the ground
landing	the area of floor at the top of a set of stairs
location	a place or position
lock	the thing that is used to close a door, window, etc., and that needs a key to open it
move house	to leave your home in order to live in a new one
neighbourhood	an area of a town or city that people live in
property	a building that someone owns
rent	to pay money to live in a building that someone else owns
steps	one of the surfaces that you walk on when you go up or down stairs
terrace	a flat area outside a house, restaurant, etc. where you can sit
view	the things that you can see from a place

Useful language

Offers, requests and asking for permission

Polite offers:

A: Is there anything we can do to help?
B: Oh no, it's all under control!
A: Let me get you something else.
B: No, it's fine, really.
A: I'll get you a green salad.
B: OK, that would be lovely. Thanks.

Asking for help politely:

A: Do you think you could give me a hand?
B: Sure.
A: Do you mind if I borrow some money?
B: Not at all. How much do you want?
A: Can I use your phone for a moment?
B: Sure. Here it is.

Asking for permission:

A: Is it OK if I just have some bread and butter?
B: No, we can do better than that.
A: May I sit here?
B: Of course. There's plenty of space.
A: Do you think I could have a glass of water?
B: Yes, of course. Let me get you one.
A: Would you mind if I opened the window?
B: Not at all. It's hot in here.

Imagining people’s feelings

A: I’m a big football fan.
B: I expect you're excited about the match this afternoon.
A: Tom’s mother is an architect.
B: That must be very interesting!
A: I'm doing three part-time jobs at the moment.
B: You must be very tired!
A: I'm going to meet my boyfriend's parents for the first time.
B: I imagine you're a bit nervous!
A: I'm planning a holiday to France.
B: That must be exciting!
A: I've just broken my tooth!
B: That must be really painful.
A: I've lost my smartphone – and I can't remember any of my friends' numbers.
B: That must be so annoying!
A: I'm learning Japanese at the moment.
B: I imagine it’s quite difficult.