4．现在进行时和一般现在时的比较
  ①暂时性动作和经常性动作
  The computer is working perfectly.
  计算机运转得很好。（暂时）
  The computer works perfectly.
  计算机运转很好。（一直如此）
  ②持续性动作和短暂性动作
  The bus is stopping.
  车停了下来。（渐渐地）
  The bus stops.车停了。（迅速） 
  ③暂时性动作和永久性动作
  She is living in the country.
  她现在住在农村。（暂时）
  She lives in the country.
  她住在农村。（永久）
  ④有感情色彩和没有感情色彩
  He is doing well at school.
  他在学校表现很好。（赞扬）
  He does well at school.
  他在学校表现很好。（一般事实）  
  
  B.过去进行时
1.过去进行时的构成
  过去进行时是由“was / were +现在分词”构成的。
  I was doing my homework at this time yesterday.
  昨天的这个时候我正在做作业。
  Were you expecting him yesterday?
  你昨天一直在等他吗？
  They were not talking when I came in.
  我进来的时候他们没在说话。

2.过去进行时的用法
  ①过去进行时的基本用法
  a.表示在过去某一时刻正在发生的动作或该动作与过去的另一动作同时发生
  I was having supper at 7:00 yesterday evening.
  昨天晚上7点我正在吃晚饭。
  She was playing the piano while I was reading the newspaper.
  她弹钢琴时我在看报。
  提示：
  当句子意思很清楚时，我们也可以把两个动词都换成一般过去时。
  We listened closely while the teacher read the text.
  老师读课文时，我们都仔细地听着。
  b.表示过去一段时间内正在进行的动作
  We were talking about you the whole morning.
  我们整个上午都在说你。
  He was watching TV at home from 3:00 to 5:00 yesterday afternoon.
  昨天下午从3点到5点他一直在看电视。
  c.表示按计划、安排过去将要发生的事
  He told me that he was going soon.
  他告诉我他很快就要走了。
  She said she was leaving for New York the next month.
  她说她下个月动身去纽约。
  ②过去进行时的特殊用法
  a.表示故事发生的背景
  It was snowing as the medical team made its way to the front.
  那支医疗小组往前线行进时天正下着雪。
  Tom slipped into the house when no one was looking.
  汤姆乘没人注意时溜进了房间。
  b.表示一个新的动作刚刚开始
  过去进行时可用来引出一个新的动作，这种用法颇有点儿像镜头转换。
  Five minutes later, he was standing at the door, smoking.
  5分钟后，他已站在门口抽着烟。
  The baby was crying hard, and suddenly the crying stopped.
  这婴儿在大声啼哭，突然，哭声停止了。
  c.用来陈述原因或用作借口
  She went to the doctor yesterday. She was having a lot of trouble with her heart.
  她昨天去看病了。她患了很严重的心脏病。
  I haven't finished my homework because I was helping my mother at home all day yesterday.
  我作业没做完是因为我昨天一直帮妈妈在家干活。
  d.与always, constantly等词连用，表示感情色彩
  The girl was always changing her mind.
  这女孩老是改变主意。
  In the past he was constantly asking me for money.
  过去他总是向我要钱。
  3．过去进行时与一般过去时的区别
  ①一般过去时往往表示某一动作已经完成，而过去进行时却表示动作在持续或未完成。
  She wrote a letter to her friend last night.
   她昨晚给朋友写了封信。（信写完了）
  She was writing a letter to her friend last night.
  她昨晚一直在给朋友写信。（信不一定写完）
  ②一般过去时表示只做一次动作，而过去进行时却表示动作反复地进行。
  She waved to me.
  她朝我挥了挥手。
  She was waving to me.
  她不断地朝我挥手。
  The boy jumped up and down.
  这男孩跳了一下。
  The boy was jumping up and down.
  这男孩不停地跳着。
  C.将来进行时
  1．将来进行时的构成
  将来进行时是由“shall/will + be +现在分词”构成的。
  Don't phone me between 8:00 and 10:00. We'll be having classes then.
  8点到10点之间不要给我打电话，我们那时正在上课。
  Will you be using your bicycle this evening?
  今晚你用自行车吗？
  She won't be having a meeting in her office at 8:00 tomorrow.
  明天8点她不在开会。
  2．将来进行时的用法
  ①将来进行时的基本用法
  a.表示在将来某一时间正在进行的动作
    I'll be taking my holiday soon.
   我不久就去度假了。
    They will be meeting us at the station.
   他们会在车站接我们的。
  b.在口语中代替will/shall do
  I hope you will be coming on time.
  我希望你按时来。
  I'll be seeing Mr. Smith tomorrow.
  我明天将见到史密斯先生。
  The minister will be giving a speech on international affairs.
  部长将就国际事务发表演讲。
  ②将来进行时的特殊用法
  a.表示原因、结果或猜测
  Please come tomorrow afternoon. Tomorrow morning I'll be having a meeting.
  请你明天下午来吧。我明天上午有个会。（表原因）
  Stop the child or he will be falling over.
  抓住那个孩子，要不他会掉下去的。（表结果）
  You will be making a mistake.
  你会出错的。（表推测）
  b.用在问句中，表示委婉礼貌
  Will you be reading anything else?
  你还要看点儿什么吗？
  When shall we be meeting again?
  我们什么时候再见面？
  c.表示稍后一点儿的安排
  The students aer studying Unit 3 this week, and next week we'll be studying Unit 4.
  这星期我们学第三单元，下周我们将学第四单元。
  My duties will end in July and I will be returning to Shanghai.
  我的任务在7月结束，之后我会回上海。