Must, have to, can’t, mustn’t and needn’t

Must and have to

A. I have to do something = it is necessary to do it. I am obliged to do it. (e.g. You can't turn right here. You have to turn left.)
A1. use do/ does/ did in questions and negative sentences (for the present and past simple) (e.g. What do I have to get a new driving licence? (not What have I to do?))
A2. can use 'have to' with 'will' and 'might/ may'. (e.g. If the pain gets worse, you'll have to go to the doctor.)

B. 'Must' is similar to 'have to'. (e.g. It's later than I thought. I must go./ I have to go.)
B1. can use 'must' to give your own opinion (e.g. to say what you think is necessary, or to recommend someone to do something). 'Have to' is also possible. (e.g. I haven't spoken to Sue for ages. I must phone her. (= I say this is necessary))
B2. use 'have to' (not must) to say what someone is obliged to do. The speaker is not giving his/ her opinion. (e.g. I have to work from 8.30 to 5.30 every day. (a fact, not an opinion))
B3. But 'must' is often used in written rules and instructions. (e.g. Applications for the job must be received by 25 April.)
B4. cannot use 'must' to talk about the past. (e.g. We had to leave early. (not we must))

C. Mustn't: You mustn't do something = it is necessary that you do not do it (so don't do it) (e.g. You must keep it a secret. You mustn't tell anyone. (= don't tell anyone))
C1. don't have to: You don't have to do something = you don't need to do it (but you can if you want) (e.g. You don't have to tell him, but you can if you want to.)

D. can use 'have got to' instead of 'have to' (e.g. I've got to work tomorrow. or I have to work tomorrow.)

Must and can't

A. We can use 'must' to say that you believe something is certain. (e.g. You've been travelling all day. You must be tired. (Travelling is tiring and you've been travelling all day, ~))
A1. can use 'can't' to say that you believe something is not possible. (e.g. You've just had lunch. You can't be hungry already. (People are not normally hungry just after eating a meal. You've just eaten, so you can't be hungry.))

B. For the past we use 'must have (done)' and 'can't have (done)' (e.g. I didn't hear the phone. I must have been asleep.)
B1. can use 'couldn't have' instead of 'can't have'. (e.g. Sue couldn't have got my message.)

Mustn't and needn't

A. You ‘must’ do something = it is necessary that you do it.
A1. You ‘mustn't’ do something = it is necessary that you do not do it (so don't do it)
A2. You ‘needn't’ do something = you don't need to do it (but you can if you like) (e.g. You can come with me if you like, but you needn't come if you don't want to. (= It is not necessary for you to come))

B. Instead of 'needn't', can use 'don't/ doesn't need to', way say 'don't need to do', but 'needn't do' (without to) (e.g. We needn't hurry./ We don't need to hurry.)

C. ‘Needn't have (done)’ (e.g. He needn't have taken the umbrella = He took the umbrella, but this was not necessary.)
C1. ‘needn't (do)’: Everything will be OK. You needn't worry. (it's not necessary) vs. Everything was OK/ You needn't have worried. (you worried, but it was not necessary)

D. I didn't need to ~ = it was not necessary for me to ~ (and I knew this at the time) (e.g. I didn't need to get up early, so I didn't.)
D1. I didn't have to ~ is also possible
D2. I needn't have done something = I did it, but now I know that it was not necessary. (e.g. I got up very early because I had to get ready to go away. But in fact it didn't take me long to get ready. So, I needn't have got up so early. I could have stayed in bed longer.)