There are three main types of conditional sentences.
（條件句主要有三類。）

（1）If it rains, I'll stay at home.
        Use if + simple present tense to talk about things which are possible or which can happen in the future.
More examples:
      If I see John I'll pass your message to him.
      Say 'hello' for me if you see John tomorrow.

（2）If I won the lottery I would buy a big house.
        Use if + simple past tense to talk about things which are imposible or not very probable. In the above example, the speaker assumes that it is unlikely that he will win the lottery.
     （用 if + 過去式表達不可能發生或發生機會很微的事。在以上例句裏，說話的人假設他贏得彩票的機會很微。）
More examples:
     If I were you I would take the offer.
     My father would kill me if he knew about it.
     If I knew the answer I would tell you.

（3）If the driver had stopped in time, the accident would not have happened.
      The real situation was that the driver did not stop in time and the accident did happen as a result. Notice the use of if + the past perfect （if the driver had stopped....） to talk about things which did not happen in the past.  Also pay attention to the construction would have happened in the other part of the sentence.
     （真實的情況是；駕車者沒有及時停車，意外亦因此而發生。留意if+過去完成式用來表達過去沒有發生的事情，另外，留意句子其他部分內 would have happened這個結構。）
More examples:
     If I have known you ex-wife was coming I would have told you. （The fact is: I didn't know she was coming so I didn't tell you about it.）（如果我早知你的前妻會來我一定會告訴你。）
      If he hadn't drunk so much the accident wouldn't have happened.（The fact is: he drank too much and the accident did happen.）（如果他沒有喝那麼多酒，意外就不會發生。）

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