' CAN ' and ' MUST '

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?

Must : (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.

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.