千万(qiān wàn) - whatever you do, be sure to; ten million in Mandarin

千万 (qiān wàn) started as a number, ten million, and that sense survives in 几千万 and in written figures. But nine times out of ten in speech it is an adverb of urgent instruction, the equivalent of "whatever you do" or "make sure you".

It leans heavily towards warnings. The commonest shapes by far are 千万别 and 千万不要 followed by the thing you must not do, and it usually turns up in imperatives or in advice about the future.

The trap is trying to use it in a statement about the past or about yourself. 我千万没去 is not Chinese. 千万 pushes on the listener; it is an appeal, not a description. For emphasis about what actually happened, reach for 真的, 确实 or 绝对 instead.