have to和must 區(qū)別:
1) 兩詞都是'必須'的意思,have to 表示客觀的需要,must 表示說話人主觀上的看法,既主觀上的必要.
My brother was very ill,so I had to call the doctor in the middle of the night.我弟弟病得很厲害,我只得半夜里把醫(yī)生請來.(客觀上需要做這件事)
He said that they must work hard.他說他們必須努力工作.（主觀上要做這件事）
2) have to有人稱、數(shù)、時態(tài)的變化,而must只有一種形式.但must 可用于間接引語中表示過去的必要或義務.
He had to look after his sister yesterday.
3） 在否定結(jié)構(gòu)中：don't have to 表示"不必"
mustn't 表示"禁止",
You don't have to tell him about it.你不一定要把此事告訴他.
You mustn't tell him about it.你一定不要把這件事告訴他.
4) must表示推測,have to不可以
1' must用在肯定句中表示較有把握的推測,意為"一定".
2' must表對現(xiàn)在的狀態(tài)或現(xiàn)在正發(fā)生的事情的推測時,must 后面通常接系動詞be 的原形或行為動詞的進行式.
You have worked hard all day.You must be tired.你辛苦干一整天,一定累了.（對現(xiàn)在情況的推測判斷）
He must be working in his office.他一定在辦公室工作呢.
過去完成時與過去時的區(qū)別：
1）一般過去時表示過去時間發(fā)生的動作或存在的狀態(tài)；
He went home yesterday.他昨天回家了.
過去完成時表示在過去某一時間或動作之前己完成的動作或狀態(tài),即“過去的過去”,常與by,before等表示過去的介詞短語連用.強調(diào)過去某一動作發(fā)生在另一動作之前時,常用過去完成時.如：
He had gone home when I got to his office.當我到他的辦公室時,他早已回家了
By the end of last term-we had learnt about 1,500 English words.到上期期末,我們已學了大約1500個英語單詞.
2)在連詞before,after引導的從句中,由于連詞本身的意義己說明主句和從句兩個動作先后發(fā)生的關(guān)系,兩個動作都可以用一般過去時,有時也可用過去完成時,強調(diào)動作的完成.如：
After he had arrived in England,Marx worked hard to improve his English.馬克思到英國后,他努力學習以提高英語水平.
He went on watching TV.after his father (had) left.他父親走后,他繼續(xù)看電視.
Marx had learnt some English before he got to England.馬克思到英國之前就學過一些英語.