Should

Should + infinitive (should watch, should go, should do, …)

You should do something = it is a good thing to do, it is the right thing to do:

Tom doesn’t study enough. He should study harder.
It’s a good movie. You should watch it.
When you play tennis, you should always watch the ball.

You shouldn’t do something = it is not a good thing to do. (shouldn’t = should not)

Tom shouldn’t go to bed so late.
You watch too much TV. You shouldn’t watch TV so much.

We often say I think ……..should…….

I think she should buy some new clothes. ( I think it is a good idea.)
I think I should go home. It’s late.
Shall I buy this coat? Yes, I think you should.

I don’t think ……should:

I don’t think you should work long hours. ( I don’t think it is a good idea.)
I don’t think we should go yet. It’s too early.

Do you think ….. should:

Do you think I should buy this scarf?
What time do you think we should leave?

Note 1: Must is stronger than should:

It’s a good film. You should go and see it.
It’s a fantastic movie. You must go and see it.

Note 2: Another way to say should is ought to:

It’s a good movie. You ought to go and see it. (= you should go)
I think Lisa ought to buy some new clothes. (= Lisa should buy)

Must / Mustn't / Don't need to

Must + infinitive (must go, must do, must watch, …)

I must do something = I need to do it.

I’m starving. I must eat something.
It’s a wonderful movie. You must watch it.
The house is very dirty. You must clean it.

For the past (yesterday, last month, …), we use had to …… (not must)

I was starving. I had to eat something. (not must)
We had to walk home last night. There were no buses. (not we must walk)

I mustn’t do something = it is necessary not to do it, it is the wrong thing to do. (mustn’t = must not)

I must hurry. I mustn’t be late again.
I mustn’t forget to take my keys.
Be happy! You mustn’t be upset.
You mustn’t touch the paintings on the walls.

Don’t need to: I don’t need to do something = it is not necessary:

You don’t need to go. You can spend the night here.
You don’t need to shout. I can hear you.

You can also say don’t have to …..:

You don’t have to do it. I can do it myself.

Compare don’t need to and mustn’t:

You don’t need to go. You can spend the night if you want.
You mustn’t go. You must stay here.

Have to

I have to do something = it is necessary for me to do it, I’m obliged to do it.

I’ll be late for class tomorrow. I have to go to the bank.
She starts work at 8 o’clock, so she has to get up at 7.
You have to pass a test before you can get a driving license.

The past (yesterday, last night, … ) is had to :

I was late for work yesterday. I had to go to the dentist.
We had to walk home last night. There were no taxis.

In questions and negatives we use do/does (present) and did (past):

Do I have to go to that party?
Does she have to attend the class?
You don’t have to go the park.
She doesn’t have to practice today.
Did you have to go there?
I didn’t have to stay there and listen to that nonsense.
What time do you have to go to the movies tomorrow?
Does he have to work every night?
Why did they have to leave so early?

I don’t have to do something= it is not necessary to do it:

I’m not working tomorrow, so I don’t have to get up early.
He doesn’t have to work very hard. His job is easy.
We didn’t have to wait very long for him. He arrived sooner than we expected.

Must and have to

You can use must or have to when you say what you think is necessary, when you give your opinion:

It’s a wonderful movie. You must see it. or You have to see it.

When you are not giving your personal opinion, use have to (not must). Compare:

Sara won’t be at work this afternoon. She has to go to the doctor. (this is not my personal opinion- it is a fact)
Sara isn’t well. She doesn’t want to go to the doctor, but I told her she must go. (this is my personal opinion)

Can and could

Can + infinitive (can do, can play, can come, …)

I can do something = I know how to do tit, or it is possible for me to do it.

I can play the piano. My brother can play the piano, too.
Sara can speak Spanish, but she can’t speak it fluently.
Can you swim?
Can you climb a tree?
I am having a party next weed, but my friends can’t come.

For the past (yesterday, last week, … ) we use could/couldn’t:

When I was a child, I could draw well.
She couldn’t speak Spanish a year ago.
I was exhausted last night, but I couldn’t sleep.
I had a party last week, but my bestie couldn’t come.

Can you …? Could you …? Can I…? Couldn’t I …?

We use can you …? or could you …? when we ask people to do things:

Can you open the window, please? or Could you open the window please?
Can you wait a minute, please? or Could you wait?

We use Can I have …? Or Could I have …? to ask for something:

(In a shop) Can I have these postcards, please? or Could I have ….?

Can I …? or Could I …? = is it OK to do something?:

Can I borrow your umbrella? or Could I borrow your umbrella?
(on the phone) Can I speak to Sara, please? or Could I speak ….?

May and might

Might + infinitive (might go, might be, might rain, …)

He might go to Canada = it is possible that he will go to Canada.

It might rain = it is possible that it will rain.

I might = It is possible that I will:

I might go to the movies tomorrow, but I am not sure. (= it is possible that I will go)
She might phone this afternoon.
Take an umbrella with you. It might rain.
Buy a ticket. You might be lucky. (perhaps you will be lucky)
I might go out.

Study the difference: