情态动词的用法要点
一． can 和could
特别说明：
（1） could 用来表示请求时，语气委婉，主要用于疑问句，不能用于肯定句，答语应用can （即：could 不能用于现
在时态的简略答语中）。如： ——Could I use your dictionary?
——Y es, you can.(否定回答可用：No, I‟m afraid not.或者是you can‟t)
表示推测时，could 不是过去式，只是语气更委婉；若是推测已发生的事或过去的情况，用can/could have done (2)
can 和be able to辨析
can(could)和be able to都可以表示能力，意思上没有区别。但can 只有现在式和过去式，而be able to则有更多的形式。如：
I ‟ve always wanted to able to speak fluent English.
Those bags look really heavy, are you sure you‟ll be able to carry them on your own?
但是，表示在过去某时的某一场合经过一番努力，终于做成了某事，通常不用could ，而用was/were able to 来表示。这时，was/were able to相当于managed to do或succeed in doing。如： After the accident it was a long time before she was able to walk again.
The fire was very big, but most people were able to escape from the building. (3) 惯用形式“cannot „too „”表示“无论怎么„„也不（过分）”。如：
Y ou cannot be too careful.你越小心越好。 惯用形式“cannot but+ 不定式（不带to ）”表示“不得不，只好”。如： I cannot but admire her determination.我不得不钦佩你的决心。
二．may 和 might
二．must 和have to
注意：表示”偏要，硬要做某事
-how old are you, madam?
-if you must know, I‟m twice my son‟s age(如果你非得知道，是我儿子年龄的两倍)
四．shall 和should
五．will 和would
特别说明：would 与used to辨析
would 可用来表示过去反复出现的动作，但不能表示过去存在的状态，所以我们不能说：“she would be a quiet girl.” 另外，would 强调过去某种特定情况下的活动，是完全过去的事情，同现在没有联系。而used to 则着眼于过去和现在的对比，隐含现在已不存在，动作或状态都可表示。Would 可以表示不规则的习惯，used to则不可。如： He used to be a naughty boy and cause trouble. I used to get up at six in the morning.
Sometimes she would take a walk in the neighboring woods.
In those days, whenever I had difficulties, I would go to Mr. Chen for help.
六．need 和dare
七．ought 的用法：
说明：should 与ought to 表示“应该”时的区别
should 表示自己的主观看法，而ought to 的语气中，含有“按道理应该„„”之意。若要反映客观情况或涉及法律义务和规定，一般用ought to。如：
Y ou should help them with their work.
Y ou are his father. You ought to get him to receive good education.
八．“情态动词+have done”用法
情态动词的用法要点
一． can 和could
特别说明：
（1） could 用来表示请求时，语气委婉，主要用于疑问句，不能用于肯定句，答语应用can （即：could 不能用于现
在时态的简略答语中）。如： ——Could I use your dictionary?
——Y es, you can.(否定回答可用：No, I‟m afraid not.或者是you can‟t)
表示推测时，could 不是过去式，只是语气更委婉；若是推测已发生的事或过去的情况，用can/could have done (2)
can 和be able to辨析
can(could)和be able to都可以表示能力，意思上没有区别。但can 只有现在式和过去式，而be able to则有更多的形式。如：
I ‟ve always wanted to able to speak fluent English.
Those bags look really heavy, are you sure you‟ll be able to carry them on your own?
但是，表示在过去某时的某一场合经过一番努力，终于做成了某事，通常不用could ，而用was/were able to 来表示。这时，was/were able to相当于managed to do或succeed in doing。如： After the accident it was a long time before she was able to walk again.
The fire was very big, but most people were able to escape from the building. (3) 惯用形式“cannot „too „”表示“无论怎么„„也不（过分）”。如：
Y ou cannot be too careful.你越小心越好。 惯用形式“cannot but+ 不定式（不带to ）”表示“不得不，只好”。如： I cannot but admire her determination.我不得不钦佩你的决心。
二．may 和 might
二．must 和have to
注意：表示”偏要，硬要做某事
-how old are you, madam?
-if you must know, I‟m twice my son‟s age(如果你非得知道，是我儿子年龄的两倍)
四．shall 和should
五．will 和would
特别说明：would 与used to辨析
would 可用来表示过去反复出现的动作，但不能表示过去存在的状态，所以我们不能说：“she would be a quiet girl.” 另外，would 强调过去某种特定情况下的活动，是完全过去的事情，同现在没有联系。而used to 则着眼于过去和现在的对比，隐含现在已不存在，动作或状态都可表示。Would 可以表示不规则的习惯，used to则不可。如： He used to be a naughty boy and cause trouble. I used to get up at six in the morning.
Sometimes she would take a walk in the neighboring woods.
In those days, whenever I had difficulties, I would go to Mr. Chen for help.
六．need 和dare
七．ought 的用法：
说明：should 与ought to 表示“应该”时的区别
should 表示自己的主观看法，而ought to 的语气中，含有“按道理应该„„”之意。若要反映客观情况或涉及法律义务和规定，一般用ought to。如：
Y ou should help them with their work.
Y ou are his father. You ought to get him to receive good education.
八．“情态动词+have done”用法