CAN : We use ‘Can ‘ to say that something is possible or that somebody has the general ability to do something . ‘ Can ‘ frequently precedes verbs of perception. Thus it indicates (a) your own ability (b) positive permission (c) perception (d) possibility etc.
- Can you lift this box? (ability)
- You can drive my car.( positive permission)
- Can you hear me well?( perception)
- He can be in the town now.(possibility)
- Children can sometimes be very trying.
- It can be very cold here even in May.
- Can I borrow your pen, please?
(1) We use ‘ Must ‘ to say that we feel /assume something. This is to show our assumption, but when we rule out or deny the probability we should use ‘ Can’t ‘ and NOT ‘ Mustn’t ‘.( Or in other words , in negative sentences we use ‘ can’t ‘ instead of ‘ mustn’t ‘ when we deal with probability aspect)
- You’ve been travelling all day. You must be tired.
- They have just had lunch. They can’t be hungry.( We don’t say, ‘they mustn’t be…)
- He must come before 8 o’ clock.( i.e based on certain assumptions, one is compelled to arrive at the conclusion that he………)
- It is 10 o’ clock now and he is never late. He can’t be coming.
- This must be the way, surely.
- This can’t be the way, surely.( We don’t say , ‘ This mustn’t be the way….)
- John has gone to Delhi. He can’t be back before Friday.
- They haven’t lived here for very long. They can’t know many people.
(2) ‘ Must ‘ indicates obligation that is considered inescapable.( here the negative is ‘ mustn’t ‘)
- You must look after your parents when they are old.
- You mustn’t hurt the helpless creature.
- I told him I must be very frank…..(OR) I told him I had to be……
Note: The past equivalent of ‘ Must ‘ is ‘ Had to ‘. However, in the past reported speech ‘Must’ can be used in place of ‘ Had to ‘.
(3) Must : when we want or intend to do something fairly soon.
- I must go and make a phone-call.
- I must meet him and convey this message today itself.
(4) Must : When we suggest that somebody should do something.
- You are a good writer. You must publish your works.
- You must come and visit me.