二、原因状语从句：1、定义： 在句中用来说明主句原因的句子叫原因状语从句。2、常用引导词 : because (因为 ), as (由于 ), since (既然 ), now (that) (既然 ) 3、时态： 原因状语从句一般都是根据实际情况选用适当的时态。通常是主过从过，主现从现。4、because, since和 as 的区别：1) because引导的原因状语从句一般放于主句的后面，because 从句位于句首时要用逗号分开，放在句末时，可不用逗号分开。because 表示直接原因，语气最强，最适合回答 why 引导的疑问句。because of 也表示原因，但它后面不接从句, 只能接名词 , 代词或动名词。注意： because 和 so 不可同时出现在一个句子里。① I do it because I like it. = I like it so I do it.② We went by bus because it was cheaper. = It was cheaper so we went by bus.③ He can’t go to school because of his illness. 2) since引导的原因状语从句一般放于主句之前表示已知的、显然的理由 (通常被翻译成 “既然 ”= now that )， 较为正式，语气比 because 弱。① Since you are free today, you had better help me with my mathematics.② Since you don't trust him, you should not employ him.③ Now (that) you are grown up, you should not rely on your parents. 3) as 引导原因状语从句时表示附带说明的“双方已知的原因”，含有对比说明的意味，语气比 since 弱，较为正式，位置较为灵活(常放于主句之前)。① As it is raining, you’d better take a taxi.② As you are tired, you had better rest.③ I went to bed early, as I was exhausted. 4) for引导的是并列句表示原因但并不说明主句行为发生的直接原因，只提供一些辅助性的补充说明，for 引导的并列句只能放于主句之后并且必须用逗号将其与主句隔开。① He could not have seen me, for I was not there. ② He seldom goes out now, for he is very old. 5. 三、目的状语从句：1、定义： 在句中用来作目的状语的句子叫目的状语从句。2、常用引导词 : so that (以便 ),in order that (以便 )3、时态： 目的状语从句一般都是根据实际情况选用适当的时态。也通常是主过从过，主现从现。4、目的状语从...
