Permission, obligation, and necessity

Modal verbs are used to express various meanings such as permission, obligation, and necessity.

In addition, there are other related verbs that can be used for expressing these meanings such as had better, be able to, be allowed to, be permitted to, and be supposed to. The choice of modal verb depends on the context and level of formality required.

1. The most common modal verbs for talking about permission and obligation are can / could, must, and should / ought to.

2. We can use should have or ought to have + past participle to talk about past events which did not happen and which we regret.

3. Had better is stronger and more urgent than should / ought to and is often used to give strong advice or a warning. It normally refers to the immediate future.
   • The negative is had better not NOT hadn’t better.

1. We also use have to to express obligations. It can be used in any tense.

2. We can also use have got to to express obligation, but it is normally used for specific occasions rather than repeated or general obligations.

1. We use need / don’t need + to + infinitive to say that something is necessary / unnecessary. You can use these forms for habitual, general, and specific necessity.

2. When we want to say that something is unnecessary on a specific occasion, we can also use needn’t + infinitive without to.
   • We use don’t need to (NOT needn’t) for habitual or general necessity, e.g. I don’t need to wear glasses, my eyesight is still good. NOT I needn’t wear glasses, …

3. When something was not necessary, but you did it, we can use either needn’t have + past participle or didn’t need to + infinitive.

4. When something was not necessary, so you did not do it, you must use didn’t need to NOT We had plenty of petrol so we needn’t have stopped, which saved time.

1. We often use be able to or be allowed to + infinitive to talk about what is possible or permitted instead of can, particularly when we want use a form which can does not have.

2. be permitted to + infinitive is used in formal situations, e.g. notices and announcements, to say what can / can’t be according to the law or to rules and regulations.

• Grammar checkpoint

  ⚠ We do not use it followed by be allowed to NOT It isn’t allowed to take mobiles into the exam room.

3. We can also use be supposed to + infinitive to say what people should or shouldn’t do, often because of rules. There is often a suggestion that the rules are not necessarily obeyed, e.g. Students are not supposed to have guests after 12.00, but everyone does.