Questions & Answers about Nie kupuj tego sera, jest stary.
Why do we use the imperfective verb kupuj here instead of the perfective kup for a one-time action?
In Polish, negative commands (telling someone not to do something) almost always use the imperfective aspect, even if you are talking about a specific, single action. So, we say nie kupuj rather than nie kup.
Why is it tego sera instead of ten ser?
The verb kupować usually takes the accusative case for its direct object (ten ser). However, when a verb that normally takes the accusative is negated with nie, its direct object switches to the genitive case.
Since ser is an inanimate object, shouldn't its genitive ending be -u instead of -a?
You might expect seru, since masculine inanimate nouns usually take -u in the genitive. However, food items, fruits, and vegetables are a common exception. They take the -a ending in the genitive singular, which is normally used for animate nouns.
Why is stary in the nominative case if we just used the genitive for the cheese?
The comma separates two different clauses. The genitive rule from the word nie only applies to the direct object of kupuj. In the second part, jest stary, there is a new hidden subject on (it) which is in the nominative case, so the adjective describing it stays nominative too.