He never forgot the lesson of 'under the plum tree, in the melon field', always being careful with his actions.

Please be careful when crossing the road.

Please be careful of cars when crossing the road.

Always be careful when crossing the road.

He always keeps a smile on his face.

He learned a big lesson from that failure.

He taught me an important lesson.

I couldn't understand his actions.

He started to act immediately.

I can't read his feelings.