as ：從屬連詞，語意較because, since弱，著重在主句，所表示的原因或理由是"附帶的"。
because ：從屬連詞，語意強，著重直接的原因或理由，所引出的從句是全句的重心，用以回答why提出的問題。
for ：并列連詞，語氣較弱，所引出的句子一般放在后面，表示附帶說明的理由或推斷理由。
since ：語意比because弱，但比as強，一般用于表示"由于大家已知的事實"。
詞組
because of
on account of; by reason of由于，因為
柯林斯高階英漢雙解學習詞典釋義
CONJ-SUBORD
（說明原因）因為，由于
You use because when stating the reason for something.
例句
He is called Mitch, because his name is Mitchell...人們叫他米奇，因為他大名叫米切爾。Because it is an area of outstanding natural beauty, you can't build on it...考慮到這一帶自然風光旖旎，你不能建在這里。
CONJ-SUBORD
（作出解釋）因為
You use because when stating the explanation for a statement you have just made.
例句
Maybe they just didn't want to ask too many questions, because they rented us a room without even asking to see our papers...也許他們只是懶得問太多的問題，因為他們都沒要求看我們的證件就把一個房間租給我們了。The President has played a shrewd diplomatic game because from the outset he called for direct talks with the United States...總統玩了一場高明的外交游戲，因為從一開始他就要求和美國直接對話。
PREP-PHRASE
因為；由于
If an event or situation occurs because of something, that thing is the reason or cause.
例句
Many families break up because of a lack of money...許多家庭由于缺錢而破裂。Because of the law in Ireland, we had to work out a way of getting her over to Britain.由于愛爾蘭的法律所限，我們不得不想辦法把她弄到英國去。
PHR-CONJ-SUBORD
僅僅因為
You use just because when you want to say that a particular situation should not necessarily make you come to a particular conclusion.
例句
Just because it has a good tune does not mean it is great music...僅憑旋律優美并不見得就是一首杰出的音樂作品。Just because something has always been done a certain way does not make it right.僅僅因為老是以某種方式做事并不意味著這么做就是對的。
英漢詞典釋義
conj.
因為
I study because I want to learn.我學習是因為我想學到知識。
[用于引導一個名詞從句，相當于 that 或 the fact that]…這一事實，…這一點原因，原因是
The reason I am here is because I want to ask for your help.我之所以到這兒來是想尋求你的幫助。
[用于省略句，表示暫不便說明詳細理由]不解釋
I think it would make her feel better because.我想它會使她覺得好受些。原因嘛，現在不說。
[口語][用于敘述原因或說明提出問題]
You must not think ill of her because she did not love you.你不該因為她不愛你就認為她這個人不好。
[口語][用于省略句,相當于 on account of being]是…的緣故
The bed was uncomfortable because too dirty.因為太臟，這張床睡得不舒服。