as if/though 句型：
    例：He spoke to me as if I were deaf. 
        他那样给我说话，就像我是聋子似的。
        The new students talked as though they had been friends for years.
        新来的学生们说话很亲热，就像多年的老朋友似的。

IF’虚拟条件句
   (1) 与过去事实相反
       例：If the man had pointed with his first finger while he was in Brunei, everyone would have thought that he was very rude.
           在Brunei，如果那人用第一个手指指着的话，那人人都会认为他很不礼貌。
   (2) 与现在事实相反
       例：If I were in your position, I would call the police immediately.
如果我处在你的位置，我会立即打电话给警察。
   (3) 与将来事实相反
       例：If I had (should have) time tomorrow, I would certainly help you.
           如果我明天有空的话，我当然会帮助你的。
   (4) 虚拟条件句中的倒装：
       例：Should it rain, the crops would grow better.
如果天下雨的话，庄稼会长得更好。
           Were I your father, I would not give you the money.
           如果我是你父亲的话，我就不会给你那钱了。
           Had it not been for the captain, the ship would have sunk
           如果不是那船长的话，那船就会沉没了。

Not --- until 句型
   (1) 陈述句 not --- until --- 直到……才
      例：Last night I didn't go to bed until 11 o'clock. 昨晚我11点钟才睡觉。
   (2) 强调句  It wasn't until…that... 直到……才…….
      例：It wasn't until yesterday that I got your letter.
          我直到昨天才收到你的来信.
   (3) 倒装句 Not until...did... 直到……才……. 
      例：Not until I began to work did I realize how much time I had wasted.
          直到我开始工作,我才意识到我已蹉跎了很多岁月.

since 句型：
   (1) Since + S + 过去式，S + 现在完成式
      例句：Since he went to senior high school, he has worked very hard.
　　        自从他上高中，他一直很用功。
   (2) It is +   --- + since   S +持续性谓语动词（表否定）
      例：It is three years since she lived here. 她已三年不住这儿了。
   (3) It is + --- + since S + 瞬间谓语动词（表肯定）
      例：It is years since I stopped smoking）．我戒烟已经数年了。