*The uses of 'Must':
1-Necessity (have/has to):
EX:Students must wear the uniform.
EX:You must obey your father.
2-Orders:
EX:You must take that medicine.(The doctor ordered the patient.)
EX:You must drive on the right.(It is a law)
EX:You must be quiet.(The teacher ordered the students.)
Note:But if we want to repeat orders for others ,we use 'have to' because we are obliged to do something.
(Outer authority)
EX:We have to drive on the right.(the taxi driver said to his friend because it is a law)
EX:I have to take that medicine.(the patient said to his mother because the doctor ordered him).
Note:There is a difference in meaning between must and have to:
EX:I must see that film.(It expresses my desire to do something)
EX:You have to go to work tonight.(The manager ordered you to go).
Note: IF you want to express necessity in the past,use had to not must and the negative is (didn't have to+inf.).
EX:He had to go to the company yesterday.It means that he already went.
EX:He didn't have to stay at home yesterday.He was busy.
Note:The negation of 'have /has to' is 'don't/doesn't have to' and they mean that there is lack of necessity.But the negation of 'must' is 'mustn't' and it is used for prohibition.They differ in meaning completely.
EX:We don't have to buy vegetables.(We already have) but there isn't a problem if you buy.
EX:You mustn't smoke in hospitals.(It is forbidden)You are not allowed to smoke.
2-Assumption (Deduction).
In this use ,must express a strong deduction that something is true based on available facts,strong evidence.
EX:He must be rich.He has a villa and a Mercedes.
EX:She must live near here.She always walk to school every day.
Note:The negation of must isn't mustn't but is can't.
EX:She can't be poor.
Note: The deduction in the past isn't 'had to; but is
'must have +p.p'.And the negation is 'can't have+p.p'.
EX:It must have rained all night .The streets are very wet.
EX:He can't have travelled abroad.I saw him an hour ago.