Must, mustn´t, don´t need - British English

1. The use of must, must not (mustn't) and need not (needn't)

The modals must, must not and need not have the same form regardless the subject. There is no ending with he/she/it.

► If you want to say the sth. is unnecessary, use need not, not must not. (The negation of mustmeans not allowed to.)

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

I need not play football. = I do not need to play football. = I do not have to play football.

I must not play football. = I am not allowed to play football.

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

Modal	Substitutes
I must play football	I have to play football.
I need not play football.	I do not need to play football.
	I do not have to play football.
I must not play football.	I am not allowed to play football.

The modal must can be used in the Simple Present only, so use the substitute have to with other tenses. The form have to has the same form regardless the subject.

Pronouns	Modal	Substitutes in the Simple Present
I, we, you, they	I must play football.	I have to play football.
he, she, it	He must play football.	He has to play football.

2. Sentences and questions with have to

2.1. have to in the Simple Present

Pronouns	Affirmative sentences	Negative sentences	Questions
I, we, you, they	I have to get upearly.	I do not have to get upearly.	Do I have to get upearly?
he, she, it	She has to get upearly.	She does not have to get up early.	Does she have to get upearly?

2.2. had to in the Simple Past

Pronouns	Affirmative sentences	Negative sentences	Questions
I, he, she, it, we, you, they	I had to get upearly.	I did not have to get upearly.	Did I have to get upearly?

2.3. Affirmative sentences with must

Modal	Substitute	Tense
I must play football.	I have to play football.	Simple Present
not possible	I had to play football.	Simple Past
not possible	I will have to play football.	will-future

2.4. Negations with must not, mustn't

Modal	Substitute	Tense
I must not play football.	I am not allowed to play football.	Simple Present
not possible	I was not allowed to play football.	Simple Past
not possible	I will not be allowed to play football.	will-future

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

Modal	Substitute	Tense
I need not play football.	I do not have to play football.	Simple Present
I do not need to play football.
not possible	I did not have to play football.	Simple Past
I did not need to play football.
not possible	I will not have to play football.	will-future
I will not need to play football.

2.6. Questions with need and have to

Modal	Substitute	Tense
Must he play football?	Does he have to playfootball?	Simple Present
Does he need to play football?
The modal must is not used in the Simple Past.	Did he have to playfootball?	Simple Past
Did he need to play football?
The modal must is not used in the will-future.	Will he have to playfootball?	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).

I need sing. (This sentence is wrong.)

http://www.englisch-hilfen.de/en/grammar/must.htm