小心

You've got to watch your step in future.

You must be careful about accepting rides from strangers.

Mind the step. / Mind you don't trip over the step.

Mind your head!

Look out! You nearly hit that wall!

Be on your guard―they always try to cheat tourists.

be extremely careful

take great care; be more careful

watch out for thieves

be careful (about) what one says

be careful not to spill the milk

She is a careful driver. / She takes great care when driving.

Handle with care. / Handle gently.

with painstaking care