Do not confuse for and since. You use for to say how long a period lasts in the past, present, or future, or how much time passes without something happening. She slept for eight hours... He will be away for three weeks... I hadn't seen him for four years. You use since to say when a period of time started. She has been with the group since it began. ...the first civilian president since the coup 17 years ago. You also use since to refer to the last time that something happened, or to how much time passes without something happening. She hadn't eaten since breakfast... It was a long time since she had been to church. You can use because, as, since, or for to give an explanation for something. Because is the commonest of these, and is used when answering a question beginning with 'why?'.You can use as or since instead of because to introduce a clause containing a reason for something, especially in writing. ...a note about The National Portrait Gallery, as the word 'Gallery' can be misleading... Since the juice is quite strong, you should always dilute it. In stories, for is sometimes used to explain or justify something. He seemed to be in need of company, for he suddenly went back into the house. Note that because is a conjunction, and is used to link two ideas within one sentence. I'm unsocial, because of my deafness.

不要混淆 for 和 since。for 用来表示过去、现在或将来的一段时间，还可表示某段时间内某事没有发生：She slept for eight hours (她睡了 8 小时)，He will be away for three weeks (他将离开 3 周)，I hadn't seen him for four years (我有 4 年没见他了)。since 用来表示某段时间的起点：She has been with the group since it began (该集团创建之初她就在那儿工作)，the first civilian president since the coup 17 years ago (17 年前政变后的第一个平民总统)。since 还可用来指某事发生的最后时间，或某段时间内某事没有发生：She hadn't eaten since breakfast (早饭后她一直没吃过东西)，It was a long time since she had been to church (她很久没去教堂了)。because，as，since，for 均可用来解释原因。because 最常用，用来回答以 why 开头的疑问句。 as 或 since 可用来代替 because 引导一个表示原因的从句，尤用于书面语：a note about The National Portrait Gallery, as the word ‘Gallery’ can be misleading (对 The National Portait Gallery 的注释，因为 Gallery 一词容易令人误解)，Since the juice is quite stong, you should always dilute it (由于果汁很浓，你必须加水稀释)。在讲述故事时，for 用来对某事作出解释或论证：He seemed to be in need of company, for he suddenly went back into the house (他似乎需要人作伴，因为他突然返回了房子里)。注意，because 是连词，用于连接一个句子中的两个意思：I'm unsocial, because of my deafness (我不善交际, 因为我耳朵聋)。