Be careful you don't trip up on the step.

你小心别在台阶上绊倒了。

《牛津词典》

You must be careful when handling chemicals.

接触化学药品必须小心谨慎。

《牛津词典》

I think you should be careful about talking of the rebels as heroes.

我认为你还是当心点好，别把叛乱分子说成是英雄。

《柯林斯英汉双解大词典》

But you must be careful. It is easy to get lost in the waves of moving sand especially during sandstorms.

VOA: special.2010.01.04

I love chemistry now. Be very careful what you say. Have any of you made that statement about hating a subject?

我现在很喜欢化学,所以你们说话的时候,可要小心咯？

麻省理工公开课 - 化学原理课程节选

So it's a way of reminding you, I've got to be careful about the types of things that I put in.

因此我这是在提醒你，一定要对输入的东西的类型,多加注意。

麻省理工公开课 - 计算机科学及编程导论课程节选

Monica: Well, Ross, you be careful now.

FORBES: Five Career Lessons From 'Friends'

But the vast majority come from other, less rigorously overlooked, stores and so the general advice is to be careful who you buy from and then be careful what access (for example to SMS privileges) you give code that you do indeed download.

FORBES: Mac and Android Users Beware: The Malware is Coming For You!

But you must be careful and pay special attention to the type of entity you are considering contributing extra hours to.

FORBES: Overview Of The New 3.8% Investment Income Tax, Part 2: Passive Activities