条件状语从句
   条件状语从句由连词if, unless (=if not) 引导。
  1. If it doesn’t rain tomorrow, we will go hiking.
  2. You will get good grades if you study hard.
  3. I will go to the party unless he goes there too.
  4. You will be late unless you leave immediately.
(=If you don’t leave immediately, you will be late.)
难点提示：用条件状语从句时要注意时态的正确使用,当主句是将来时的时候,从句要用一般现在时.
   He will not leave if it isn’t fine tomorrow.
一般将来时，一般现在时
   They are going to have a picnic if it doesn’t rain next week.
一般将来时，一般现在时
原因状语从句
   由连词because, since, as引导, 也可由for, now that 等词引导
   I didn’t go to school yesterday because I was ill.
   Since everybody is here, let’s begin our meeting.
   As you are in poor health, you should not stay up late.
   I asked her to stay to tea, for I had something to tell her.
难点――because , since , as , for,辨析
   1）because语势最强，用来说明人所不知的原因，回答why提出的问题。当原因是显而易见的或已为人们所知，就用as或 since。
   Since /As the weather is so bad, we have to delay our journey.
   2）由because引导的从句如果放在句末，且前面有逗 号，则可以用for来代替。但如果不是说明直接原因，而是多种情况加以推断，就只能用for。
   He is absent today, because / for he is ill.
   He must be ill, for he is absent today.
   3) as和for的区别：
通常情况下，as引导的从句在主句前，for引导的从句在主句后。例：
   As the weather is cold, I stay at home.（同义句）
   I stay at home, for the weather is cold.