Questions & Answers about Дайте мне, пожалуйста, глоток квасу.
Why is the word for kvass spelled квасу here instead of the usual genitive кваса?
This is the partitive genitive, often called the second genitive. For certain uncountable mass nouns like food and drink, you can use the ending -у or -ю instead of -а or -я to talk about a specific quantity or portion. Because глоток (a sip) is a portion, we use квасу to specify a sip of that drink.
Would it be grammatically wrong to just say глоток кваса?
Why doesn't the word глоток change its ending after the verb Дайте?
The verb Дайте (give) requires its direct object to be in the accusative case. Because глоток is an inanimate masculine noun, its accusative form is identical to its basic nominative dictionary form.
Why do we use the perfective verb дайте here instead of the imperfective давайте?
When making a specific, one-time polite request for someone to hand you something, Russian almost always uses the perfective aspect. The imperfective давайте is mostly used to mean 'let us' do something, or for continuous and repeated actions.