Talk about obligation and ability

This lesson brings together two topics that stayed incomplete in A2. The first is ability and success: can (A1 #20) has too few forms. The second is rules: to must, have to (A2 #10) and should (A2 #11) we add "that is the rule", "that is allowed" and "that is not necessary".

be able to — ability in any tense. can has only two forms: can and could. It has no future, no perfect, no form after another verb — there be able to does the work.

In the present people usually say can: I can swim sounds more natural than I'm able to swim.

Could do or managed to do? In the past English keeps apart two meanings that many languages give with one verb "can". could is ability in general, over some period of time: I could read when I was four. Success on one particular occasion is was able to or managed to. managed to stresses that it was difficult, but it worked.

In the negative there is no such difference; couldn't always works: I couldn't find my keys this morning. With the verbs of perception see, hear, could is fine about one occasion too: I could hear music next door.

In a positive sentence was able to and managed to are almost the same, but managed to speaks openly about difficulty and effort. That is why but and in the end so often come with it: It took three hours, but in the end we managed to fix it. If there was no difficulty, people usually just use the Past Simple: I finished the report on Friday.

be supposed to — that is the rule, that is what was agreed. The obligation does not come from the speaker or from the law, but from rules, an arrangement or other people's expectations. You're supposed to take your shoes off in the house. — that is the custom. I'm supposed to be at work at nine. — that is what was agreed. Often this form lets you hear that in reality things go differently: I'm supposed to be working, but I'm watching football. In the past it almost always means "it should have happened, but it didn't": The train was supposed to arrive at six. — but it came at seven. not supposed to is a soft prohibition by the rules, softer than mustn't: You're not supposed to park here. supposed here is an adjective, so be is always needed: I'm supposed to, Were we supposed to…?

Compare it with have to. I have to be at work at nine — I have no choice: if I'm late, there will be trouble. I'm supposed to be at work at nine — that is the rule, but it sounds as if the rule can be broken and perhaps I will break it. That is why be supposed to is good when you explain why something did not go to plan.

be allowed to — you may, it is permitted. The permission is given by someone else: rules, the law, parents, the boss. allowed is the third form of allow "to give permission", so in its structure this is the passive, familiar from the notice Pets aren't allowed (A2 #19): not "I permit", but "I am permitted". be changes with the person and the tense, and allowed to + the verb stays the same.

can or be allowed to? In the present people often talk about permission with simply can: You can park here after six. be allowed to stresses that the permission comes from the rules or from another person, and so it sounds more exact when rules are announced or asked about: Are we allowed to bring food into the museum? In the future and in the perfect can has no form, and be allowed to is necessary — just like be able to: Will we be allowed to park here? About the past the line is the same as between could and managed to: could is permission in general, over a long time (As a child I could watch TV until nine), was allowed to is permission on one occasion (Yesterday we were allowed to leave early).

supposed to or allowed to? Both forms are about rules, but they say different things. supposed to is what you should do, allowed to is what you may do. In the negative the difference is in strength:

don't need to — there is no need. This is the same as don't have to (A2 #10): you can choose not to do it. need here is an ordinary verb with to, so the negative, the question and the past are built with do: You don't need to come tomorrow. Does she need to bring anything? We didn't need to wait. Don't confuse it with mustn't and not allowed to — "it is forbidden": You don't need to come — you are free not to come; You aren't allowed to come — coming is forbidden.

The whole scale in one example:

Don't say this. ✗ We didn't need come so early → ✓ We didn't need to come so early. didn't need is an ordinary verb, and to is needed after it.

Check yourself. could or managed to: "it was difficult, but in the end I caught the train"? What does The film was supposed to start at eight mean? How do you say that the rules at your work don't let you and your colleagues use your phones?