21. through, across
　　through穿越空间，across在…上穿过 through the forest, across the desert
　　22. on the way, in the way
　　on the way在前往…的路上，in the way挡路 The chair is in the way.
　　23. above, on, over
　　above在上面，不接触，on在上面，接触，over在正上方 fly over the hill
　　24. until, not…until
　　until到…为止， not…until直到…才（常跟点动词连用）I waited until 3:00. He didn't come until 3:00.
　　25. besides, except, except for
　　besides除了…还（包括在内）except除了（不包括在内），except for整体…除了某一点以外 The composition is good except for a few spelling mistakes.
　　26. weather, if
　　当是否解时，只有在宾语从句中二者可以互换，其余都用weather，当如果解时用if I don't know if/whether he will come. If he comes, I'll let you know.
　　27. and, or
　　and并且，or或者，否则，常用于否定句中 I don't like apples or bananas.
　　Hurry up and you'll catch the bus. Hurry up or you'll miss the bus.
　　28. because, since, as, for
　　原因由强到弱为：because, since/as, for. 在句中的位置如下：1)…because… 2) Since/as…, … 3) …, for… Since I was ill, I didn't go.
　　29. when, as, while (表时间)
　　when从句动词点动词，持续性动词均可，as重在表示动作同时发生，伴随进行，while从句动词为持续性动词 While I slept, a thief broke in.
　　30. the same…as, the same…that
　　the same…as和…一样的（相似但不同一）， the same…that 同一物体 This is the same pen that I used yesterday. (同一支笔)
　　31. as well, as well as
　　as well也，常放于句末，和and连用表示既…又；as well as并列连词，不但…而且… He is a professor, and a writer as well.
　　32. such…as, such…that
　　such…as像…样的，such…that如此…以至于 He is not such a fool as he looks like. He is such a good student that all the teachers like him.
　　33. because, because of
　　because连词，连接两句话，because of介词短语，后接词或短语 He didn't go to school because of his illness.
　　34. in order that, in order to
　　表目的，in order that后接句子，in order to后接动词原形 I got up early in order to catch the first bus. I got up early so that I could catch the first bus.
　　35. for example, such as
　　for example一般只列举一个，such as列举多个例子 I have been to a lot of American cities, such as New York, Atlanta and Chicago.
　　36. used to, would
　　表过去常常，和现在相对应用used to，不提现在用would I used to get up early, but now I don't.
　　37. All right. That's all right. That's right.
　　All right. That's all right. 当好吧解时，可以替换；当不客气，没关系解时只能用That's all right. That's right. 那是对的 ---Sorry. --- That's all right.
　　38. such…that, so…that
　　当如此…以至于解时，such…that修饰名词，so…that修饰形容词或副词，但名词前面如果有many, much, little, few修饰用so…that，不用such that
　　so many people that… such a lovely boy=so lovely a boy
　　39. so + be (have, can, do)+主语， neither(nor) + be (have, can, do)+ 主语
　　也一样，肯定用so…否定用neither (nor) ---I can't play tennis. ---Nor can I.
　　40. Shall I…? Will you…?
　　Shall I…? 征求对方意见或向对方请示，意为我能…吗？ Will you…? 请求或建议对方做某事，意为你愿意…吗？Will you help me? Yes, I will.