Be careful when you cross the road!

当你过马路时小心点。

Be careful when you cross the road.

当你过马路时要小心。

You must be careful when you cross the road.

你过马路时务必要小心。

You must be as careful as you could (=as careful as possible) when you cross the road.

当你过马路的时候，你一定要尽可能的小心。