MUST - HAVE TO AND NEED

If you want to say the sth. is unnecessary, use needn't - not mustn't.
Be careful: The negation of must means not allowed to.

I must play football. = I have to play football.

I needn't play football. = I don't need to play football. = I don't have to play football.

I mustn't play football. = I'm not allowed to play football.

You can use must only with Simple Present benutzt. If want to use it with other tenses, you need the form have to. This form is not the same regardless the subject. Look the following table.

Modal	have to in Simple Present
I must play football.	I have to play football.
You must play football.	You have to play football.
He/She must play football.	He/She has to play football.
We must play football.	We have to play football.
They must play football.	They have to play football.

Affirmative sentences with must and have to

Modal	Form	Tense
I must play football.	I have to play football.	Simple Present
Do not use must in Simple Past.	I had to play football.	Simple Past
Do not use must in will-future.	I will have to play football.	will-future

Negations with must not, mustn't

Modal	Form	Tense
I must not play football. I mustn't play football.	I am not allowed to play football. I'm not allowed to play football.	Simple Present
Do not use must not in Simple Past.	I was not allowed to play football. I wasn't allowed to play football.	Simple Past
Do not use must not in will-future.	I will not be allowed to play football. I won't be allowed to play football.	will-future

Negations with need not, needn't, don't need, don't have to

Modal	Form	Tense
I need not play football. I needn't play football.	I do not have to play football. I don't have to play football.	Simple Present
I do not need to play football. I don't need to play football.
Do not use need not in Simple Past.	I did not have to play football. I didn't have to play football.	Simple Past
I did not need to play football. I didn't need to play football.
Do not use need not in will-future.	I will not have to play football. I won't have to play football.	will-future
I will not need to play football. I won't need to play football.

Questions

Modal	Form	Tense
Must he play football?	Does he have to play football?	Simple Present
Does he need to play football?
Do not use must in Simple Past.	Did he have to play football?	Simple Past
Did he need to play football?
Do not use must in will-future.	Will he have to play football?	will-future
Will he need to play football?

The form need is not used as a modal, this is a main verb. This form cannot be put before another verb.

I needn't sing. (Here needn't is a modal. The main verb is sing.)

We need a new computer. (Here need is a main verb).