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what is the difference between "can have' and 'may have'. An item can have many sub items OR an item may have many sub items - which is correct?
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I would say in
"An item can have sub items",
"Can have" expresses that there is an ability of having several sub items...
This item can have sub items (but it doesn't have to have sub items)

...while in
"An item may have sub items",
"May have" rather expresses incertitude.
This item may have sub items (but I don't know exactly, you should check it out).
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Make yourself a favor a make a search at this site with
may can have
or
may can
(top right search box)

There are already very good threads on this.
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Comments  
If I were to speak of either the ability to have sub items or the possibility of having sub items, I should say: An item can have many sub items.

If I were to speak of either the permission to have sub items or the possibility of having sub items, I should then say: An item may have many sub items. can = either ability or possibility ("Can you lift 100 pounds?", "Can this be the end?") may = either permission or possibility ("May I leave now?", "He may win the scholarship")
I have a question regarding the use of "can have" or "may have": when you are in a restaurant, and you want a glass of wine (for example), is it correct to say "Can I have a glass of wine, please?" or "May I have a glass of wine, please?".
I personally think it is wrong to use "may" because I would be asking the waiter for permission to have a glass of wine, and that is not right. But I have recently had a dispute with some friends over this, and I'd like to be sure.

Thank you.
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 Marius Hancu's reply was promoted to an answer.