Politeness using negative questions

Hi,
I have a lot of doubts about negative questions. I know you can use them when you are impatient, complaining, etc: "Can't you just go away?"
You can also use them when you are expecting a positive answer: "Don't you have a sister called Amy? I think you do." - "Didn't you buy some yesterday? I think you did."

Now the problem is... in Italian we sometimes use negative questions with yet another meaning: it's practically the same as asking a normal question, but you sound a little more polite, because you aren't asking directly for something, but you ask a negative question. In this case the effect is the opposite of expecting a positive answer, because you are now actually expecting a negative one, so you don't sound like you are insisting and you sound a little more polite.

My pen doesn't work anymore... Don't you have a spare one for me?
This cake is really delicious! ...Can't I have another slice, please?
Very interesting. I like your new product. ...By the way, don't you have a brochure?

I need some advice, thanks

PS: I just realized what I said is not true. We don't always say that in Italian, it depends, only with certain verbs.

My pen doesn't work anymore... Don't you have a spare one for me?
This cake is really delicious! ...Can't I have another slice, please?
Very interesting. I like your new product. ...By the way, don't you have a brochure?

The closest pattern I can think of in English that might just possibly get that shade of meaning across is this:

My pen doesn't work anymore.  You wouldn't happen to have a spare one for me, would you?
This cake is really delicious.  I don't suppose you'd let me have another, would you?
Very interesting.  I like your new product.  You wouldn't happen to have a brochure, would you?

The formula is You wouldn't happen to ... or I don't suppose [you'd be able to give me ... / I could have ... / ...], with the appropriate tag question at the end.

These formulas might be considered excessively indirect, timid, and/or self-effacing by any but the most conservative of Americans, but they'll work if you want to be very indirect and polite.

CJ

Asking in the negative when you "anticipate" the positive can be seens a "pushy" or arrogant or self-centered. You are assuming that what you want will be given to you, not politely requesting it. That is removed when you use the "you wouldn't happen to have" phrasing as suggested by CJ. It's softened tremendously as well by your "by any chance."

"Can't I have..." sounds like you're saying "I should be able to, and only out of politeness am I pretending that it's really your choice."

"Don't you have..." sounds like you're saying "You darn well should, so if you answer me in the negative, I'm going to know what a loser you are."

Hi again,
I think you guys are right. maybe now I understand. I just read some stuff on the net about negative questions, plus one of Jim's posts, and this "idea" I had seems the right one:

Kooyeen wrote:

Idea! Maybe that only works with modals and with requests, not permissions? So couldn't you, wouldn't you, won't you, can't you... are used for politeness, but can't I, couldn't I, don't you, aren't you... only sound like you're insisting, which is the opposite of being polite?

Negative questions are possible with modals, but they are not going to sound more polite, but a little less polite instead, because they imply a positive answer is expected and so they actually sound a little more demanding. -> Wouldn't you like a cookie? Couldn't you answer my question?
When there are no modals, a positive answer is still expected, and negative questions are not used to ask a neutral or polite question.

What made me change my mind was an article written by someone who had a non-native (Russian) boss who sounded really rude because of his negative questions, which he thought they were a polite way to ask everything. If I hadn't found out about this, I would have end up like him, LOL.

I still have a problem: I said asking negative questions with modal verbs is still considered "polite", even though it sounds a little bit more demanding (Could you...? ---> Couldn't you...? // Would you...? ---> Wouldn't you...? // etc.)
I'm afraid (but not sure) there are some exceptions.
First exception: not all modals. Can might be an exception (Can you...? ---> Can't you?) and he negative might not be accepted among the "polite" expressions. Can you give me one? ---> Can't you give me one?
Second exception: this might not work with the first person I or we (Can I...? Could I...? ---> Can't I...? Couldn't I?) and the negative wouldn't sound polite anymore. Could I go out? ---> Couldn't I go out?

Sorry if this thread is a mess... this is very different in Italian, and I'd like to avoid sounding rude or annoying someone without wanting to.
Thanks a lot in advance.

Mario, don't you have a corkscrew here in the kitchen, by any chance?

On this one in particular, I would leave off the "by any chance".  Other than that, given the situation you describe, the negative question sounds fine to me, provided you mean by it that you think there really is a corkscrew in the kitchen.  (Maybe you've seen it there before, and Mario knows you've probably seen it there.) 

But (and again, without "by any chance") if you're 'flying blind' and have no idea whether there might be a corkscrew in the kitchen or not, this is not the right question to ask.  In that case it can sound like you are saying Mario is a fool for not having a corkscrew in the kitchen.  Don't you have a corkscrew?  You're not very prepared, are you?

CJ