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。

5. neither/ either/ both

(1) neither作代词是对两者都进行否定，意思为“两者都不”，作主语时谓语动词用单数.

Neither of the boys is from England.这两个男孩都不是来自英国。

I know neither of them. 他们两个我都不认识。

neither用作形容词，也修饰单数名词，意思与作代词时相同;用作连词时，一般与nor搭配，表示 “既不…也不”。作主语时，谓语动词也遵循就近原则。

She neither ate nor drank yesterday. 她昨天既不吃也不喝。

Neither he nor we play football on Sundays. 他和我们星期天都不踢球。

(2) either作代词时，是指两者中的任意一方,(两者之)每一个，故作主语时谓语动词用单数.

Either of the books is new.这两本书任何一本都是新的.

She doesn’t like either of the films.这两部电影她都不喜欢.

either作形容词, 用来修饰单数名词,意思与作介词时相同.

Either school is near my home. (这两所学校中的)任何一所学校都离我家很近.

Either question is difficult.两个问题(中的任何一个)都难.

either作连词时,一般与or搭配,表示两者选其一，意思是“不是…就是”。作主语时，谓语动词遵循就近原则。

Either he or I am right.不是他就是我是对的。

Either my sister or my parents are coming to see me.不是我姐姐就是我父母要来看我。

(3) both作代词时，指的是所涉及到的“两者都”, 故作主语时,谓语动词用复数形式。

I like both of the stories.这两个故事我都喜欢。

Both of my parents are teachers.我父母两人都是老师。

both作形容词时，用来修饰两者,意思与作代词时相同.

Both his arms are hurt.他的两只胳膊都受伤了。

Both these students are good at English. 这两个学生都擅长英语。

both用作连词时，多与and搭配，表示“既…又, 不仅…而且”, 作主语时,谓语动词仍用复数形式。

Both piano and violin are my bobbies.钢琴和小提琴都是我的爱好。

They study both history and physics. 他们既学历史，又学物理。

6. find/look for/ find out

(1) find强调找的结果，意思是“找到”。此外还有“发现，发觉”的意思，后可接宾语从句。

Jim couldn’t find his hat.吉姆找不着帽子了。

Have you found your lost keys? 你找到丢失的钥匙了吗?

He found the lights were on along the street.他发现沿街的灯都亮了