What's the difference between 细心, 当心, and 小心 which all mean careful?
当心 is a verb
细心 is an adjective
小心 is both an adj & a verb

I often hear both 小心 and 当心 used in the command/imperative form. How do I know when to use which?

Question

What's the difference between 细心, 当心, and 小心 which all mean careful?
当心 is a verb
细心 is an adjective
小心 is both an adj & a verb

I often hear both 小心 and 当心 used in the command/imperative form.  How do I know when to use which?

Answer

细心: be careful;
小心:cautious;
当心: look out!

Answer

Hello Toffler,

I'll tell you about the usage of the one I know well:

小心 ! 
When you just tell someone 'xiaoxin' ,that is a command ,the imperative form as in " Be careful!".

It is used as well as an adjective as follows:
- to be careful with something like fragile objects for example:
小 心 对 待 某 物
xiaoxin dui dai mou wu
to be careful when dealing with certain objects.

- 小 心 的 [xiăoxīn de] = cautious

Answer

细心is mean careful; circumspect; attentive 

当心is  to be cautious; to be careful; to look out; to take care; to watch out for; to beware of ,can't be attributive

小心is 当心 but could bu attributive 小心的人but not 当心的人

Answer

细心，心细一个意思，心很细说明这个人办事很认真：举例：生病的时候，老婆细心的照顾我；
当心和小心意思差不多，就像BE CAREFULL！这个意思