because是连词，其后接原因状语从句或回答以why开头的问句；because of是复合介词，其后接名词、代词或动名词或以what引导的名词性从句。

从词性上看

1.because是连词，其后接句子；because of是复合介词，其后接名词、代词、动词，what从句等。

(1)I didn't buy it because it was too expensive.

我没有买是因为它太贵了。

(2)He is here because of you (that).

他为你(那事)而来这里。

(3)He lost his job because of his age.

由于年龄关系他失去了工作。

(4)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。

(5)He knew she was crying because of what he had said.

他知道她哭是因为他说的话。

2.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。

(1)He didn't come because he was ill.

他因病未来。

(2)He didn't come because of his illness.

他因病未来。

(3)I can't stop fighting because I have a family.

我不能因为有个家就停止斗争。

(4)I can't stop fighting because of having a family.

我不能因为有个家就停止斗争。

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。

(1)It is because he loves you.

那是因为他爱你。

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。

(2)His absence was due to the rain.

他因雨缺席。

(3)He was absent because of the rain.

他因雨缺席。

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。

(4)It is just because of money.

那只是因为钱的原因。

(5)That was because of his sickness.

那是因为他生病的原因。