Resolved questions
what is the difference between 千万 and 一定 ？

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.