千万 (qiān wàn) on the surface seems to just mean "thousand ten thousand," but it actually means "to make certain." It is used when you want to command someone or remind someone to do something (like: "don't forget to clean your room!") It really adds a kick to your sentence!
千万 usually starts the imperative sentence/phrase (also called command sentence/phrase), and it is usually paired with 要, 不能, 别 or similar types of words. You can use 千万 with sentences that are commanding someone not do do something, or you can tell them to do something.
The following examples use 千万 to command someone not to do something.
The following examples use 千万 to command someone to do something.