• Can is used to say that somebody knows how to do something. * can 表示懂得做：◆Can you play the piano? 你会弹钢琴吗？ It is also used with verbs of seeing, noticing, etc. 该词亦与表示看见、注意到等的动词连用：◆I can hear someone calling. 我听见有人在呼叫。and with passive infinitives. 并与不定式的被动形式连用：◆The podcast can be downloaded here. 播客可在此下载。
• Can or be able to are used to say that something is possible or that somebody has the opportunity to do something. * can 或 be able to 表示某事情有可能或某人有机会做某事：◆Can you/are you able to come on Saturday? 你星期六能来吗？
• You use be able to to form the future and perfect tenses and the infinitive. 用 be able to 构成将来时、完成时和动词不定式：◆You'll be able to get a taxi outside the station. 在车站外可搭乘出租车。◆I haven't been able to get much work done today. 我今天未能干多少工作。◆She'd love to be able to play the piano. 她很希望能弹钢琴。
• Could is used to talk about what someone was generally able to do in the past. * could 表示过去通常能做：◆Our daughter could walk when she was nine months old. 我们的女儿九个月大就会走路了。
• You use was/were able to or manage (but not could) when you are saying that something was possible on a particular occasion in the past. 关于在过去特定情况下可能的事用 was/were able to 或 manage，但不用 could：◆I was able to/managed to find some useful books in the library. 我总算在图书馆找到了一些有用的书。◆I could find some useful books in the library. In negative sentences, could not can also be used. 否定句也可用 could not：◆We weren't able to/didn't manage to/couldn't get there in time. 我们未能及时赶到那儿。 Could is also used with this meaning with verbs of seeing, noticing, understanding, etc. 亦可用 could 加表示看见、注意到、明白等的动词表示此义：◆I could see there was something wrong. 我发觉出事了。
• Could have is used when you are saying that it was possible for somebody to do something in the past but they did not try. 表示过去有可能做某事但没有做，用 could have：◆I could have won the game but decided to let her win. 我本可以赢得那场比赛，但还是决定让她赢了。

• Can and cannot (or can't) are the most common words used for asking for, giving or refusing permission. * can 和 cannot（或 can't）是表示请求、给予或拒绝许可的最通用词：◆Can I borrow your calculator? 我可以借用你的计算器吗？◆You can come with us if you want to. 如果你愿意可以跟我们一起来。◆You can't park your car there. 你不能在那儿停车。
• May (negative may not) is used as a polite and fairly formal way to ask for or give permission. * may（否定式 may not）用以表示礼貌的正式请求或给予许可：◆May I borrow your newspaper? 把你的报纸借我看行吗？◆You may come if you wish. 你想来的话可以来。 It is often used in official signs and rules. 该词常用于正式标志和规定：◆Visitors may use the swimming pool between 7 a.m. and 7 p.m. 访客从早 7:00 到晚 7:00 可在游泳池游泳。◆Students may not use the college car park. 学生不得在学院停车场停车。 The form mayn't is hardly ever used in modern English. 现代英语几乎不用 mayn't。

• The modal verbs are can, could, may, might, must, ought to, shall, should, will and would. Dare, need, have to and used to also share some of the features of modal verbs. * can、could、may、might、must、ought to、shall、should、will 和 would 均为情态动词。dare、need、have to 和 used to 亦具有情态动词的某些特性。
• Modal verbs have only one form. They have no -ing or -ed forms and do not add -s to the 3rd person singular form. 情态动词只有一种形式，没有 -ing 或 -ed 形式，第三人称单数也不加 -s：◆He can speak three languages. 他会说三种语言。◆She will try and visit tomorrow. 她明天将设法去参观。
• Modal verbs are followed by the infinitive of another verb without to. The exceptions are ought to, have to and used to. 情态动词后跟不带 to 的动词不定式，但 ought to、have to 和 used to 例外：◆You must find a job. 你必须找到一份工作。◆You ought to stop smoking. 你应当戒烟。◆I used to smoke but I gave up two years ago. 我过去抽烟，但两年前就戒了。
• Questions are formed without do / does in the present, or did in the past. 疑问句现在时不用 do/does，过去时不用 did：◆Can I invite Mary? 我可以邀请玛丽吗？◆Should I have invited Mary? 我本该邀请玛丽吗？
• Negative sentences are formed with not or the short form -n't and do not use do / does or did. 否定句用 not 或简约式 -n't，不用 do/does 或 did。

You will find more help with how to use modal verbs at the dictionary entries for each verb. 情态动词的不同用法可参考本词典里各情态动词词条。