How to Say “Be Sure To / Whatever You Do” with 千万 (qiānwàn)

千万 (qiānwàn) is written with the characters for “thousand” (千) and “ten-thousand” (万), and some sources gloss it as “to make certain,” but neither the number nor “make certain” tells you how it really works. 千万 is an emphatic adverb that lives only inside a command or a plea. It means “be sure to / whatever you do / for heaven's sake / by all means,” and its whole job is to add urgency, often anxious urgency, to telling someone to do, or not do, something.

Structurally, 千万 fronts the imperative and leans on a helper word: (Subject +) 千万 + 别 / 不要 / 不能 / 要 + Verb Phrase. It's most common in the negative: 千万别迟到！(“whatever you do, don't be late!”), 你千万不要相信他 (“be sure not to trust him”), 千万不能再搞砸了 (“you absolutely mustn't mess it up again”). It also works in the positive with 要: 过马路千万要小心 (“be sure to be careful crossing the road”), 千万要记得关空调 (“be sure to remember to turn off the AC”). Notice you don't drop the helper: in a full sentence it's 千万要锁门, not just 千万锁门.

Here is the part your instinct is right about. 千万 feels like “definitely / certainly,” and that's the force it carries, but it is not the “definitely” that describes a fact. You can only 千万 an order or a plea, never a statement of what is or will be true. “He'll definitely come tomorrow” is a prediction about a fact, so it's 他明天一定会来, and 他明天千万会来 is simply wrong. If you can rewrite your sentence as “for heaven's sake, (don't) …” it's 千万; if you mean “surely / certainly” about something that's the case, you want 一定 or 肯定.

That's exactly the split laid out in the comparison below: 千万 versus 一定 (yīdìng). Both can lean on 要 to push someone (你千万要来 / 你一定要来), but 千万 is an emotional plea that only ever appears in commands, while 一定 is the broader “definitely / certainly / must” that covers both firm requirements and plain predictions (他一定会赢, “he'll definitely win”). When you're begging or warning someone, reach for 千万; when you're stating what's certainly so, reach for 一定.