从词性上看

because 是连词，其后接句子；because of 是复合介词，其后接名词、代词、动名词、what 从句等。如：

I didn’t buy it because it was too expensive. 我没有买是因为它太贵了。

He is here because of you (that). 他为你(那事)而来这里。

He lost his job because of his age. 由于年龄关系他失去了工作。

We said nothing about it, because of his wife’s being there. 因为他妻子在那儿，我们对此只字未提。

He knew she was crying because of what he had said. 他知道她哭是因为他说的话。

注意：because of 之后可接 what 从句，但不能接 that 从句或没有引导词的句子。如：

他因病未来。

误：He didn’t come because of he was ill.

误：He didn’t come because of that he was ill.

正：He didn’t come because he was ill.

正：He didn’t come because of his illness.

比较以下同义句：

他离开了这家公司，是因为老板在会上讲的话。

正：He left the company because of what the boss said at the meeting.

正：He left the company because of the thing that the boss said at the meeting.

我不能因为有个家就停止斗争。

正：I can’t stop fighting because I have a family.

正：I can’t stop fighting because of having a family.

从用法上看

because 所引导的从句除用作原因状语外，还可用作表语。如：

It is because he loves you. 那是因为他爱你。

而复合介词 because of 所引导的介词短语通常用作状语而不用作表语。如：

他因雨缺席。

误：His absence was because of the rain.

正：His absence was due to the rain.

正：He was absent because of the rain.

但是有时若主语是代词(不是名词)，也可以用 because of 引出的短语作表语。如：

It is just because of money. 那只是因为钱的原因。

That was because of his sickness. 那是因为他生病的原因。