 Must 
Must, like have to (L2U2L4), means "it is necessary." We also use must to talk about rules. For example: You must do your homework. Sometimes, must is used to make a suggestion or to show that we understand something. For example: You must see that movie.
You didn't eat anything all day. You must be hungry.
Like all modals, must:
Must has no past or future form. We use have to instead (for more on have to see L2U2L4). For example: You didn't have to come yesterday, but you must come today.
You must clean your room now, or you will have to clean it tomorrow.
Must not / mustn’t is a special case. See below. Sometimes we use must in a question, but it is very formal. In questions, do + have to is more commonly used. For example: Must I come with you? (formal)
Do I have to come with you? (common)
To make a Wh- question, add the Wh- word at the beginning of the Yes/No question. For example: When must we be there? (formal)
When do I have to be there? (common)
When we want to say that something is not necessary, we use don't have to in its different forms (L2U2L4). We don’t use must not or mustn’t (see below for explanation of mustn’t). For example: She must do her homework,
but she doesn't have to do it now.
Mustn't Must not / Mustn't means that something is not allowed. For example: You mustn't make noise here!
You mustn’t touch anything.
When speaking, the first t in mustn’t is silent. Polite Expressions The modals would, may, can and could are used in polite expressions. We use them when we want to ask for something politely, when we want to offer help or when we ask for permission. For example: Would you please open the door for me?
May I bring you something to drink?
The expression would like is a more polite way of saying want. For example: I would like (I'd like) to buy these books, please.
Would you like to dance?
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