Agree with upstairs in most cases，“千万” is used with “not & never .etc”， E.G. You must not repeat what you have seen today. 你今天看到的事千万不要告诉别人。 You must keep secret of what you have seen today. 你今天看到的事一定要保密。

My view is that: 千万: For God's sake (as Elenore said usually + don't)......; whatever happens I hope....; whatever happens you'd better..... 比如说: 这次考试我千万要考好。 Whatever happens I have gotta pass my exam this time. 一定: certainly, of course, definitely, without a doubt, must be. 千万is used to show the speaker's intention with emphasis whereas 一定 is often used to express a truth/fact.

when you are drunk ,you should not drive car(千万) please dont drive car,,, when you are drunk ,you must not drive, you cant drive(一定) when you want someone not to do it,just express your wish. 千万 when you ask someone not to do it,you order him not,,,一定, just to express different,,tone, 千万 Is light... 一定 is more stronger.

千万 used in negative sentences

To put it simply, both 千万 and 一定 mean "must" have to do something; their only difference is in their degree of action. 千万 has a lighter tone than 一定, which almost sounds assertive.