How to use MUST? :
MUST, HAVE TO and OUGHT TO, like SHOULD, are used to express obligation or duty.
I must get up at five tomorrow.
You have to catch the first train.
We ought to respect our teachers.
HAVE GOT TO may be used in stead of HAVE TO in conversation.
I‘ve got to go now.
MUST and OUGHT TO have no past forms.
Past obligation is usually expressed by HAD TO.
I had to get up at five this morning.
He had to cook his own meals when his wife was away.
There is a slight difference in meaning between MUST and HAVE TO. It would be useful to note the difference. MUST is preferred if the obligation comes from the speaker. HAVE TO is used to suggest that the obligation comes from somewhere else.
You must come to the office at fine tomorrow. (This is an order from me. I want you to come at nine.)
You have to come to the office at nine tomorrow. (The boss requires you to come at nine.)
I must try to lose weight. (It is my own idea)
I have to try to lose weight. (The doctor has told me to lose weight)
OUGHT TO mainly expresses moral obligations. It says what would be good for somebody.
You ought to help her.
I ought to give up smoking.
NEED is normally used in questions and negatives.
Need I come again? (Is it necessary for me to come again?)
You needn’t do it now. (It isn’t necessary for you to do it now.)
The negative forms MUSTN’T and NEEDN’T have quite different meanings. We use MUSTN’T to say that something is not allowed, that there is necessity not to do it. We used NEEDN’T to say that there is no necessity to do something.
You mustn’t go there. (Don’t go there.)
You needn’t go there. (You can if you like but there is not necessity.)
We may use DON’T HAVE TO (or HAVEN’T GOT TO) instead of NEEDN’T. But NEEDN’T is more usual.
In the above examples, NEED is used as a special verb and had no –s and -ed forms.
NEED may also be used, like an ordinary verb, with do, does, did.
Do I need to come again?
He doesn’t need to go to the post office.
I didn’t need to buy the book.
Look at the last example again and compare.
I didn’t need to buy the book (It was not necessary for me to buy the book and I didn’t buy it)
I needn’t have bought the book. (It was not necessary for me to buy the book but I bought it.)
Note that MUST and OUGHT TO are also used to express strong probability.
He must be seventy now.
She started at four so she ought to be in Bangalore by now.
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How to use MUST?
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