In structures like these, the subject is usually I. Sometimes you use an impersonal structure beginning with it or you. For example, instead of saying I ought to mention that he had never been there, you can say It ought to be mentioned that he had never been there.
importance: must
5.215 If you feel strongly that what you are saying is important, you use must.
I must apologize to you.
I must object.
It must be said that he has a point.
importance: should and ought to
5.216 If you feel that it is important or appropriate that something is said, you indicate that you are going to say it by using should or ought to.
I should explain at this point that there are two different sorts of microscope.
It should also be said that I learned a great deal from the experience.
I ought to stress that this was not a trial.
Perhaps I ought to conclude with a slightly more light-hearted question.
politeness: can and could
5.217 If you want to say something during a discussion, you can indicate politely that you are going to say it by using can.
Perhaps I can mention another possibility.
If I can just intervene for one moment…
If you want to be even more polite, you use could.
Perhaps I could just illustrate this by mentioning two cases that I know of personally.
Perhaps I could just ask you this…
5.218 You also use can and could when you are mentioning an opinion or a way of describing something.
Can suggests that you approve of the opinion or description.
Such behaviour can be a reaction to deep emotional upset.
Could is more neutral.
You could argue that this is irrelevant.
You could call it a political offence.
approvaclass="underline" may and might
5.219 May and might can also be used to mention an opinion or a way of describing something.
May suggests that you approve of the opinion or description. It is more formal than can.
This, it may be added, greatly strengthened him in his resolve.
Might also suggests that you approve of the opinion or description. You use might when you think there is a possibility that the person you are talking to will disagree with you.
You might say she’s entitled to get angry.
That, one might argue, is not too terrible.
politeness: should and would
5.220 If you are stating an opinion of your own, you can show politely that you are going to state it by using should.
I should think it would last quite a long time.
Would is used in a similar way, but is less common.
I would guess it may well come down to cost.
Expressions used instead of modals
5.221 Several ordinary verbs and fixed expressions are used to express the same attitudes and ideas as modals. These verbs and expressions are explained in the following paragraphs. Each group of paragraphs corresponds to an earlier section in the chapter dealing with the use of modals in a particular type of situation.
saying whether something is possible
5.222 Be able to and be possible to can be used instead of can and could to say whether or not something is possible.
The subject of be able to and be unable to usually refers to a person or group of people, but it can refer to any living thing. It can also refer to something organized or operated by people, such as a company, a country, or a machine.
The subject of be possible to is always the impersonal pronoun it.
5.223 If you want to say that it is possible for someone or something to do something, you can use be able to.
All members are able to claim travelling expenses.
The college is able to offer a wide choice of subjects.
You use be able to with a negative to say that it is not possible for someone or something to do something.
They are not able to run fast or throw a ball.
5.224 You also use be unable to to say that it is not possible for someone or something to do something.
I am having medical treatment and I’m unable to work.
We are unable to comment on this.
5.225 You also use be possible to with it as the subject to say that something is possible. You usually use this expression to say that something is possible for people in general, rather than for an individual person.
It is possible to insure against loss of earnings. Is it possible to programme a computer to speak?
If you use be possible to to say that something is possible for a particular person or group, you put for and a noun phrase after possible.
It is possible for us to measure his progress.
It’s possible for each department to support new members.
You use be possible to with a negative to say that something is not possible.
It is not possible to quantify the effect.
5.226 You can also use be impossible to to say that something is not possible.
It is impossible to fix the exact moment in time when it happened.
It is impossible for him to watch TV and talk.
5.227 To change the tense of be able to, be unable to, be possible to, or be impossible to, you simply change the form of be to an appropriate simple form.
The doctor will be able to spend more time with the patient.