4. since/ for

(1) since用于完成时态，既能用作介词，也能用作连词，后常接时间点，意思是“自从”。

He has been a worker since he came into this city.

自从他来到这个城市，他就是工人了。

I have never seen him since we last met in Shanghai .

自从我们上次在上海见过之后，我再也没见过他。

since作连词，还有“既然”的意思。

Since you are interested in it, just do it. 既然你对它感兴趣，那就做吧。

You can have fun now since you’ve finished your work.

既然你已经做完了功课，就开心玩会儿吧。

(2) for用于完成时，用作介词，后常接一段时间，意思是“经过…”。

I have learned English for five years.我已经学了五年英语了。

They have waited for you for 30 minutes.他们已经等了你三十分钟了。

for也可以用作连词，但意思是“因为”。

They missed the flight for they were late.他们由于完到了而误了航班。

He fell ill for many reasons.他由于多种原因病倒了。

9. except/ besides

(1) except是指不包括后面所提人或物在内的“除了”，可以理解为“撇开…不谈”,表示两部分的不同。

Everyone is excited except me.

除我以外的每个人都很激动。(他们激动，而我却不激动)

All the visitors are Japanese except him.

除他以外的所有游客都是日本人。(其他人是日本人，可他不是)

(2)besides是包括后面所提人或物在内的“除了”，可以理解为“除之外…还、除之外…又”，表示两部分的相似性。

Twenty-five students went to the cinema besides him.

除他以外，还有25个学生去看了电影。(他和另外25人都去了)

We like biology besides English.

除了英语外，我们还喜欢生物。(生物和英语都喜欢)

besides还可用作副词，意思是“此外;而且”，常用于句首或句尾。

He is a great thinker, and besides, he is a politician.

他是一名伟大的思想家，除此以外，他还是一位政治家。

They encouraged me, and they supported me with money, besides.

他们不仅鼓励我，而且与我以金钱上的支持。

10. keep doing/ keep on doing

(1) keep doing指的是连续地、坚持不断地做某事，中间不间断。

It kept blowing for a whole day.刮了一整天风了。

The temperature keeps dropping.温度持续下降。

(2) keep on doing是指反复坚持做某事，但动作之间略有间隔。

They have kept on writing to each other for many years.他们已经互相通信多年了。

After drinking some water, he kept on talking.喝了一些水后，他坚持讲话。

11. seem/ look

(1) seem一般着重于以客观迹象为依据，意思是“似乎、好象、看起来…”。

The baby seems to be happy.婴儿看上去似乎很高兴。

He seemed to be sorry for that.他似乎为那件事感到抱歉。

seem能与to do结构连用，而look不能。

It seems to rain. 似乎要下雨了。

They seemed to have finished their work.他们似乎已经完成了工作。

在it作形式主语的句型中只能用seem。

It seems that he is quite busy now.他现在看起来很忙。

It seems to us that there is nothing serious.在我看来没什么大不了的。