1. because 是連詞，其后接句子；because of 是復合介詞，其后接名詞、代詞、動名詞、what 從句等。如：
I didn't buy it because it was too expensive. 我沒有買是因為它太貴了。
He is here because of you (that). 他為你(那事)而來這里。
He lost his job because of his age. 由于年齡關系他失去了工作。
We said nothing about it, because of his wife's being there. 因為他妻子在那兒，我們對此只字未提。
He knew she was crying because of what he had said. 他知道她哭是因為他說的話。
注：because of 之后可接 what 從句，但不能接 that 從句或沒有引導詞的句子。如：
他因病未來。
誤：He didn't come because of he was ill.
誤：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.
比較以下同義句：
他離開了這家公司，是因為老板在會上講的話。
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.
我不能因為有個家就停止斗爭。
正：I can't stop fighting because I have a family.
正：I can't stop fighting because of having a family.
2. because 所引導的從句除用作原因狀語外，還可用作表語。如：
It is because he loves you. 那是因為他愛你。
而復合介詞 because of 所引導的介詞短語通常用作狀語而不用作表語。如：
他因雨缺席。
正：His absence was due to the rain.
正：He was absent because of the rain.
但是有時若主語是代詞(不是名詞)，也可以用 because of 引出的短語作表語。如：
It is just because of money. 那只是因為錢的原因。
That was because of his sickness. 那是因為他生病的原因。