Breakdown of Onko teillä kuitti? Ei, meillä ei ole kuittia.
olla
to be
me
we
te
you (plural/formal)
ei
no / not (en, et, ei, emme, ette, eivät)
Questions & Answers about Onko teillä kuitti? Ei, meillä ei ole kuittia.
Why does the word for receipt change from kuitti in the question to kuittia in the answer?
This is the main rule for negative possession! In a positive sentence (or question) about having one whole thing, you use the basic form (kuitti). But when you say you don't have something, the thing you don't have must be in the partitive case (kuittia).
Why do we use ei ole here instead of emme ole for "we do not have"?
In Finnish "have" sentences, the verb is always in the "it is" form (on). This is because literally you are saying "at us is not a receipt". Therefore, the negative form is always ei ole (it is not), even when the people who don't have the thing are "we" (meillä) or "you" (teillä).
What does the -ko at the end of Onko do?
The -ko (or -kö) is a question particle used to make yes/no questions. By attaching it to the verb on (is) and putting it at the start of the sentence, it turns "You have a receipt" (Teillä on kuitti) into "Do you have a receipt?" (Onko teillä kuitti?).
I know teillä means "you have", but does it mean singular or plural "you"?
Teillä is the "at" form of te, which means either plural "you" (a group of people) or formal "you" (speaking respectfully to one person). So this sentence works perfectly whether you are asking a group of people or a single customer in a formal setting.