注意する

You usually point to the danger or focus using the particle ni. For example, kuruma ni chuui suru means 'watch out for cars.' If you want to tell someone else to be careful, use the te form: chuui shite! It is very flexible. You can use it as a verb or a noun.

Use it when there is a specific thing to watch out for. Are you walking on an icy sidewalk? Use it. Is there a tricky question on a test? Use it. You will see it on signs everywhere in Japan. 'Watch your head' or 'Mind the gap' both use this phrase. It is also used when a teacher or boss gives you a 'warning' or a 'reminder.'

Don't use it as a general 'take care' when saying goodbye. For that, ki o tsukete is much better. 注意する feels a bit more like a focused warning. Also, don't use it if you are apologizing for a mistake. It sounds like you are telling yourself to focus, not saying sorry to the other person. Keep it for active situations.

Japan is a culture of 'safety first.' You will find chuui signs on almost every corner. From 'Beware of crows' to 'Don't lean on the doors,' the culture values preventing trouble before it happens. Being 'attentive' is seen as a sign of a mature, responsible adult. It shows you care about your surroundings and other people's safety too.

The most common version you'll hear is chuui shite kudasai (please be careful). In a more casual setting, friends just say chuui shite ne! If a boss gives you a stern talking-to, you might say you were chuui sareta (given a warning). It’s a word that scales perfectly from a friendly nudge to a serious official notice.