雖然都表原因，各自側重不同。

【例句】：She spoke quietly because she did not want to wake up the baby.

【翻譯】：她小聲得說話，因為不想吵醒孩子。（直接的因果，不想吵醒孩子導致說話聲音小）

【例句】：Because I am disappointed, I want to go.

【翻譯】：因為我失望了，所以我想離開。

【例句】：Don’t tell me bad things in the morning as it sets the mood for the rest of the day.

【翻譯】：大早晨的，別告訴我壞事，因為早晨的心情決定了一整天的心情。

【例句】：As it is raining outside, you’d better stay at home.

【翻譯】：外面在下雨，你最好還是呆家裡吧。

【例句】：They are expensive, since their quantity is small.

【翻譯】：物依稀為貴。

【例句】：Since people can do many things at home via internet, it is not necessary for them to find a job outside.

【翻譯】：既然人們可以在家透過網路做很多事，所以他們無須出去工作。

【例句】：He must be at home, for it is raining.

【翻譯】：他一定在家，因為下雨呢。 下雨不是必須在家的原因，卻是推測的依據。

【例句】：I want to see you, for I miss you.

【翻譯】：我想見你，因為我想你。

because口語常用，後面接句子，說明直接原因是什麼。the car crashed because the driver was careless.直接原因是因為駕駛員不小心，所以汽車撞壞了。

for後可接句子和短語，表示推測的原因。for做介詞後面跟名詞或名詞性片語。 for做連詞連線句子，表推測原因，或原因對主句的補充說明。I was angry with him for being late .我很生氣，因為他遲到了（for後面加的是短語）It must have rained,for the ground is all wet.(for後面加句子表示推測原因)天一定下過雨，因為地上全溼了。根據地上溼了推測下雨

since和as說明的原因理由不是直接的，重要的而是大家都知道的，一般放句首。翻譯成既然。As you are tired, you"d better rest.既然你累了，那就休息吧。since light travel faster than sound ,we see lightening before we hear the thunder.因為光比聲走的快，所以我們先看到閃電，後聽到雷

because和for強調“為什麼”，說話人希望大家關注原因

Because引表示直接原因，for表示間接原因，或者推斷的根據。

for引導的原因狀語從句一般只能在句中。

如：

Kate spoke quietly because she didn’t want Ali to hear.

Kate 平靜地說話，因為她不想讓Ali聽到。

I’m sure it rained last night, for the ground is wet.

我確信昨晚下雨了，因為地面是溼的。

二詞差別不大，但since的理由比as更充分一些。

Since everyone is here, let’s start.

既然大家都來了，我們就開始吧。

I really want Ellen to come as I want to hear about her trip to France.

我真的很想讓Ellen 來，因為我想聽一聽她去法國的旅行。

例句:(注意觀察下面例句中，主從句的先後順序，以及逗號的使用。)

Because I opened Lisa’s letter, she’s angry with me.

因為我打開了麗莎的信，她生我的氣。

Lisa is angry with me because I opened her letter.

麗莎因為我打開了她的信而生我的氣。

The days are shorter and shorter, for it’s December.

白天越來越短了，因為是十二月。

As Mike was in Spain, he decided to watch Flamenco dance.

Mike在西班牙時，他決定去看弗拉門戈舞。

Mike decided to watch Flamenco dance as he was in Spain.

Mike決定在西班牙觀看弗拉門戈舞。

We had a video conference (影片會), since half of our colleagues were in the US.

我們有一個影片會議，因為我們一半的同事都在美國。

Since half of our colleagues were in the US, we had a video conference.

因為我們一半的同事都在美國，所以我們開了一個影片會議。