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1.because 可用来回答why提出的问题；可以引导表语从句，而as则不行;
2.because 表示的是必然的因果关系，语气最强,通常表示重要的信息；as/since所表示的是对已知事实提供理由，而不表示直接原因，since比较正式;
Example:
—Why are you late? 你为什么迟到了？
—Because I woke up late in the morning. 因为我早上起床晚了。
Example: This is because I love you.
这是因为我爱你。
Example: I did it because he told me so.
我这么做是因为他告诉我的。
Example: As he was still busy, we went shopping without him.
由于他很忙，我们就没有跟他一起去购物。
3. 句中的for为一并列连词,可用来表示因果关系,通常用来引导一事后想到的原因,更类似于置于括号内的说明性文字由于它的这些特征,因此在使用时不可直接用其引导一原因状语从句,或像原因状语从句那样置于句首（used to introduce the reason for sth mentioned in the previous statement ）
如不可说
*For I was feeling quite hungry, I decided to stop and have lunch.
而应说
I decided to stop and have lunch, for I was feeling quite hungry.
我决定停下来吃午饭,因为我觉得非常饿。
4. as引导的原因状语从句表示一种直接因果关系。as一般放在句首，相较于because语气较弱，较口语化。 Example: As both my friends and I were interested in Korean culture, we chose South Korea as our destination. 因为我和我的朋友们都对韩国文化感兴趣，所以我们选择去了韩国。 Example: As we hadn’t seen each other for a long time, I made a journey to Beijing to visit him. 因为我们很久没见面了，所以我去了趟北京去见他。 Example: As Harry is the chosen boy, he must be the one to defeat Tom. 因为哈利是被选中的男孩，所以必须由他来击败汤姆。 |