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The following parts are taken from PEU:1

122.2 common or typical

We often use can to say what is common or typical.

Scotland can be very warm in September.

Ann can really get on your nerves sometimes.

345.2 'general' possibility: can/could, not may/might

We normally use can/could to say that things are possible in general: people are able to do them, the situation makes them possible, or there is nothing to stop them. May/might are not used in this way.

These roses can grow anywhere. (NOT ••• These roses may grow anywhere.)

Can gases freeze? (NOT ••• May gases freeze?)

In those days, everybody could find a job. (NOT ••• In those days, everybody might find a job.)

339.10 another use of may/might: typical occurrences

In scientific and academic language, may is often used to talk about typical occurrences - things that can happen in certain situations.

A female crocodile may lay 30-40 eggs.

The flowers may have five or six petals, pink or red in colour.

Children of divorced parents may have difficulty with relationships.

With this meaning, might can be used to talk about the past.

In those days, a man might be hanged for stealing a sheep.

These explanations made me puzzled. Since they can both relate to typical occurrences, is it possible to substitute can/could for may/might in PEU 339.10 if the context is not written in scientific and academic language?

1. PEU = Michael Swan's, Practical English Usage.

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The word typical is, I think, ill-chosen here. What the writers are saying at 122.2 might be better expressed thus;

We often use can to signify that an eventuality does occur—not always, not necessarily even usually, but from time to time.

At 339.10 there are really two different uses which should be distinguished:

May is used, particularly in academic and scientific contexts, to signify that an outcome is possible but not necessary:
Children of divorced parents may have difficulty with relationships.

May is also used in academic and scientific contexts to describe a range within which possible outcomes lie:
A female crocodile may lay 30-40 eggs.
The flowers may have five or six petals, pink or red in colour.

Either of these may uses can be expressed with can in colloquial registers. Colloquial can has largely (but not entirely) replaced may.

And (as you doubtless know) these do not exhaust the possible uses of either may or can.

  • THX! Do the examples imply 30-40 eggs/5 or 6 petals is the upper range for a female crocodile's eggs/the flowers and the normal range will be a little lower than that? – Kinzle B Apr 25 '14 at 13:27
  • @ZhanlongZheng By default these mean a range of from 30 to 40 eggs and either five or six petals, either pink or red - no other quantities or colors. But the sad fact is you can never count on a writer being sufficiently explicit: these writers may mean "up to 30 or even 40" and "up to five or six". That however is a problem with the writers, not with the language. – StoneyB Apr 25 '14 at 13:40
  • So by default, do these examples themselves preclude the possibility of any outcomes outside of the given range? – Kinzle B Apr 25 '14 at 14:08
  • @ZhanlongZheng That's what you'd expect it to mean: the female crocodile will not lay fewer than 30 or more than 40 eggs (if she lays any eggs at all - you may reasonable infer that the writer is speaking only of female crocodiles who have mated). – StoneyB Apr 25 '14 at 16:03
  • Looking back, I feel "In those days, a man might be hanged for stealing a sheep." a little bit odd. As you said, the word typical is ill-chosen here. So is the case with might, I suppose, right? But we don't use "might + infinitive" alone to suggest a possibility in the past except when using it in a reported clause. We should use "might pp" or "could pp" instead. What do you think? @StoneyB – Kinzle B May 25 '14 at 15:37
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The nuance of may and can is easy to understand but when it's about asking a question. When these verbs form a sentence (and not a question) they serve differently.

If I apply the general rule to this particular question, can talks about the capability of someone/something whereas may talks about the things that happen naturally. This having said, in the latter case, the subject is not active, has not put their efforts.

A female crocodile can lay 30-40 eggs - shows her capacity
A female crocodile may lay 30-40 eggs - shows natural occurrence, her capacity is not emphasized

Since scientific and academic research is more based on natural occurrences the book is suggesting using may over can which might refer to someone's capability or efforts to have that thing - This is my opinion.

I addressed your major concerned about replacing/interchanging can and may for that section. However, I guess rest is clear as it is.

Good read about may and can is here.

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