What is the function of “お” and “で” in this expression and what does “かい” means?
お - an honorific prefix. You prefix certain nouns to make the sentence more respectful. It has no meaning of its own.
The example sentence is thus a very polite and formal one - “Tomorrow, what would you buy at the store?”, said with respect.
で - a particle for “in”, “at”, and so on. But there is also に with similar meaning which has different usage and connotation.
で is usually used when there’s some active participation involved - an “action particle”, so to speak.
For example, “I studied at school” would be "(わたしは)学校で べんきょう した。 (学校 = school), whereas “There is a car at the school” would be "学校に くるまが ある”. For the same “at”, either で or に is used, and they are not interchangeable.
かい - a connective form of かう(買う), meaning “buy”. かいますか is かい + ます(polite ending) + か(interrogative ending).
お- prefix has been added to みせ (store/shop) to make the word sound more respectful as userstk said, since this word doesn’t normally have the prefix お- in neutral polite speech.
Please note that often words expressed in daily life use this prefix too, but like a notation of familiarity. eg. おちゃ (tea), おかね (money), おさけ (sake), etc.
Polite masu-forms of userstk’s examples:
ie. “I studied at school” = (わたしは)学校 で べんきょう しました。
”There is a car at the school" = 学校 に くるまが あります。
Yes, I realize I left out the う. I just went back and added it in.
About the polite endings, I agree it is a better form for beginners. I get too impatient and sometimes too space conscious to use the longer form, if it’s not the main part of discussion
Now that you mentioned it, the original poster can see both versions, which is good.
Thanks
You don’t have to have any doubts about the expression. It is absolutely correct.
I think Truzen is using “doubt” to mean “question”.
We often have some quirky language habits and funny word choices, especially with foreign languages.
@Truzen: it’s probably better not to use the word “doubt” because it might sound like “you doubt the correctness of the sentence”. You are just asking a question about the sentence, so “question” is probably a better word.