Will you turn on the radio, please?
Would you turn on the radio, please?
May you turn on the radio, please?
What are the differences between all these way of asking a request?
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Will you pass me the salt, please?
Polite, but be careful with intonation.
Stressing the words you, and please while looking daggers at your interlocutor will make the request sound abrupt, and accusatory.
The words; you, pass, and salt, should be stressed lightly and naturally. The words pass me should sound as if they were linked.
Would you pass me the salt, please?
If I had to write a polite request on paper, this is the one I would choose. I don't know exactly why, but would sounds more pleading to my ears, as if you're really asking someone for an act of kindness. Maybe it's Dr Seuss's Green Eggs and Ham (1960) that drummed it into my head.
Would you like them
in a house?
Would you like them
with a mouse?
Would you? Could you?
In a car? Eat them! Eat them!
Here they are.
Which brings me to could, which the OP forgot to add to the list
Could you pass me the salt, please?
Again, very polite. There's hardly any difference between the modal verbs could and would, in terms of politeness.
Now, some very stuffy grammarian or pedant might object, and point out that can refers to ability, and speakers should say may I; yawn, yawn, yawn ... Native speakers continually use the words can and could in favours and requests.
May you pass me the salt, please?
This is not idiomatic. It's not common and it sounds as if I am asking him or her permission to pass me the salt. This request borders on pretentious sounding, and also close to weird. Far, far better would be to ask:
May I have the salt, please?
Extremely polite, it scores a straight 10 on the polite-o-meter.
The first one is the least polite and the last one is the most polite in my opinion. Someone may have been annoyed for a while and losing patience if he asks in the first sentence's way.