When you are unsure about what is expected of you in a situation, you can ask about obligations. 不确定在某种情境下该做什么时可以询问应履行的义务：

• ◆What time do we have to be home? 我们必须在什么时候回到家？
• ◆ Are we supposed to show our ID cards? 我们需要出示身份证吗？
• ◆ Is it necessary to apply for a visa? 有必要申请签证吗？
• ◆ Is there a legal obligation to wear a bike helmet here? 在这里骑自行车有没有法律规定要戴头盔？

Necessity and Obligation 必要和义务

• Must and have (got) to are used in the present to say that something is necessary or should be done. Have to is more common in NAmE, especially in speech. * must 和 have (got) to 用于现在时，表示某事有必要或应该做。have to 较常用于美式英语，尤其是口语中：◆You must be home by 11 o'clock. 你必须在 11 点之前回家。◆I must wash the car tomorrow. 我明天必须洗汽车。◆I have to collect the children from school at 3 o'clock. 我得在 3 点钟到学校接孩子。◆Nurses have to wear a uniform. 护士必须穿制服。
• In BrE there is a difference between them. Must is used to talk about what the speaker or listener wants, and have (got) to about rules, laws and other people's wishes. 在英式英语中，两词之间有差异。must 是基于说话者或听话者的主观意愿，have (got) to 关乎规定、法律和他人的愿望：◆I must finish this essay today. I'm going out tomorrow. 我今天一定要完成这篇论文，因为我明天要出去。◆I have to finish this essay today. We have to hand them in tomorrow. 我今天得完成这篇论文，因为我们明天必须交。
• There are no past or future forms of must. To talk about the past you use had to and has had to. * must 无过去和将来形式。表示过去用 had to 或 has had to：◆I had to wait half an hour for a bus 我得等半小时的公共汽车。 Will have to is used to talk about the future, or have to if an arrangement has already been made. 说将来的事用 will have to，如果已作好安排亦可用 have to：◆We'll have to borrow the money we need. 我们需要的这笔钱只好去借了。◆I have to go to the dentist tomorrow. 我明天得去看牙医。
• Questions with have to are formed using do. 带有 have to 的疑问句由 do 构成：◆Do the children have to wear a uniform? 孩子们必须穿制服吗？ In negative sentences both must not and don't have to are used, but with different meanings. Must not is used to tell somebody not to do something. 在否定句中，用 must not 和 don't have to，但二者含义不同。must not 用于告诉某人不要做某事：◆Passengers must not smoke until the signs have been switched off. 指示灯未熄灭之前乘客不许抽烟。 The short form mustn't is used especially in BrE. 缩略形式 mustn't 尤用于英式英语：◆You mustn't leave the gate open. 你一定不要让大门敞开着。 Don't have to is used when it is not necessary to do something. 表示没有必要做某事用 don't have to：◆You don't have to pay for the tickets in advance. 你不必预付票款。◆She doesn't have to work at weekends. 她周末不用上班。

Certainty 肯定

• Both must and have to are used to say that you are certain about something. Have to is the usual verb used in NAmE and this is becoming more frequent in BrE in this meaning. 表示肯定用 must 和 have to 均可。have to 通常用于美式英语中，在英式英语中也越来越常用于此义：◆He has (got) to be the worst actor on TV! 他无疑是最糟糕的电视演员！◆(BrE) This must be the most boring party I've ever been to. 这无疑是我参加过的最无聊的聚会。 If you are talking about the past, use must have. 说过去的事用 must have：◆Your trip must have been fun! 你这次旅行一定很开心吧！

• 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。