Modal verbs – negatives forms

Modal verbs: negative forms
positive	negative
can (used for possibility) It can happen; everything's possible.	can't (cannot) It can't happen; it's impossible.
can (used for permission) Can I smoke here? ∼ Yes, you can.	can't (cannot), mustn't You can't smoke here. You mustn't smoke here.
can (used for ability) I can play the guitar.	can't (cannot) I can't play the guitar.
must, have to (used for obligation) You must be there at 8 o'clock. You have to be there at 8 o'clock.	don't have to, needn't You don't have to be there at 8 o'clock. You needn't be there at 8 o'clock.
must (used for personal opinion, certainty) He must be here somewhere – his car is outside.	can't (cannot) He can't be here – his car's not outside.
might, may (used for personal opinion, certainty) He might/may be late today – there's a lot of traffic on the roads.	may not, might not He might/may not get here on time – there's a lot of traffic on the roads.
should (used for weak obligation / advice) You should stop smoking because it's unhealthy.	shouldn't (should not) You shouldn't smoke so much – it's unhealthy.