当心(dāng xīn) - to watch out, to be careful, mind in Mandarin

当心 (dāng xīn) is the warning word. It fronts a sentence as a bare alert (当心！), takes a hazard as its object (当心台阶, 当心那条狗), or takes a clause with 别 for the thing you want avoided (当心别感冒了).

It is close to 小心 (xiǎo xīn) and the two swap freely in most sentences. The difference is slight: 当心 leans towards an immediate, external danger and is the one you see stencilled on signs, while 小心 also covers being careful in a general, temperamental sense.

The trap is the tone on 当. Here it is first tone, dāng; the fourth-tone dàng is a different word meaning to pawn or to treat as. And note it is a verb, not an adjective, so it takes no 很: 你要当心 is right, 你很当心 is not.