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can / could

The speaker says that something is possible or impossible because of the law, physical or mental ability, logic or science, custom or morality... or a combination of these.

For a table of examples with remote forms, click here .
To see tables of the frequency of words used with "could" , click here.

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may / might

... for possibility.More subjective than "can/could"

It may/might (not) rain

It may/might (not) be raining

It may/might (not) have rained

It may/might (not) have been raining

... for permission. The speaker gives permission, ...or refuses it

You may (not) sit here.

(might is not used here, it is too remote)

... can/could or will/would are much more frequent for possibility;
but for permission...

May we leave now?

May I have another piece of your delicious cake?

... are natural.

The remote form is very formal...

Might I ask your name?

Might I see you next week?

... and, well, remote!

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Subjective view of the idea of necessity = I say it is necessary that...


I must see you ( I need to see you)

She must be in the office ( ...this is logical, necessarily true)

She must be in the office by 9:00 (... that's the rule)

You mustn't tell anyone (... it is necessary not to tell anyone)

It must have been very hot! ( is logical that it was hot)


Must is a modal verb. It is subjective, and

"I must go" is very similar to " I have to go"

.. but "I mustn't go" = I say it is necessary not to go
"I don't have to go" = it is not necessary to go

"Do you have to go?" is not subjective, but tentative and... talk about the past
or other remote more objective
attitudes "have to/had to" is used, so ...

" I had to go " = it was necessary
" I didn't have to go" = it was not necessary
" Did you have to go?" = was it necessary?

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shall / should
With "will" , the speaker says that something is inevitable,
with "shall" there is more of the speaker's personal involvement.

You shall be there at 6:30 ( I order you )

We shall not warn you again (... that is a promise)

Shall we go out?

For "should" Lewis (LTP) gives 5 distinct groups:


" If he should be late..."/"Should he be late..." = If he is late...

"Should you not contact us..." = If you do not contact us...


"You should watch that video" = I think you will enjoy it
"You should have taken a taxi" = ...but you did not take a taxi

"You should not watch so much TV" = I recommend you do not watch so much
"You should not have eaten that fish" = It was a bad idea

"We suggest that passengers should carry only 5kg" = It is best if...


"I should think he's in Paris" = In my estimation
he is there

"You shouldn't have any problems" = I don't think you will...


How should I know? = Don't ask me

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 will / would
The graphic below shows how the modal verb goes together with other auxiliary and main verbs, and it also shows the approximate frequency of these combinations with " would".

• " would " is the most frequent modal verb ( BNC )

• " will " is also very common, but it has another written form ( -'ll) and a great range of attitude and meaning depending on intonation and stress in speech.


"Context" includes the relationship between the speaker or listener, writer or reader.

For example, there is a form called "future perfect" or "future anterior" which is usually presented to learners as a " completed action in the future", predicted by the speaker.
• a medical expert, interviewed by a journalist about a recently successful operation to separate conjoined twins said " It will have been a very difficult procedure".

• a press officer, talking about the latest photographs of a train accident : " You will have seen the photos of the damage ..."

• a teacher, writing on a web page :" Most students of English will have learned that "will" is "the future form in English"

...these forms are frequent, a search of the BNC with the term " will + have + vbn " will give examples of the form with past, current and future references. They use "will " to emphasise that they are giving an opinion about current and past fact.

Most common uses are for
an offer:- " I'll help you " / "I'll make some coffee"

• a promise or a threat. You decide which, depending on the situation:- " I'll see you tomorrow" / " I will always love you" / " You will never see me again" / " It will be a cold day in hell before I help you again!"

• negotiation:- " I'll pay you an extra 3% if you can deliver on Friday" / " For an extra 5% I'll deliver it tomorrow"

prediction- science/ astrology/ fortune telling:- " There will be rain tomorrow in the west and north" / "Capricorns will have a good year"

a speaker's view of habit/liklihood:- "He'll be at home in bed now" / " She'll be working all next week"

"-'ll"- relaxed future marker ( what is any view of the future, if not an opinion?) Mixed in with any other prospective language forms e.g. be + -ing , be going to , first form verbs which may be independent of any modal. Find your own examples.

• restaurant/ shopping:- "I'll have fish" / " I'll take it"

authority/announcement :- " The Court will rise" / " You will not use your computer for playing games" / " The Queen will not visit Margate this year"

It's wrong to ignore the social and personal implications of these uses. And there can be a crossover between the above areas. Check with a search on the BNC or with the COBUILD corpus. As noted above, a search for any of the modal perfect forms (a modal + have) will throw up some interesting results, and that's a promise.

• " would" is the remote form of " will"...

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