Be careful in all things.

凡事要小心。

The fellow is as sharp as a razor. Be careful in your dealings with him.

这家伙厉害得很， 你和他打交道时要特别小心。

You must be careful in future.

你今后必须小心。

Be careful in money matters.

在钱方面要特别小心。

You should be careful in the sun.

在阳光下要当心。