Breakdown of Ja kupuję chleb, a ty kupujesz mleko.
ja
I
ty
you (singular)
a
and / whereas (contrast)
kupować
to buy
Questions & Answers about Ja kupuję chleb, a ty kupujesz mleko.
Why didn't chleb change its ending if it is the direct object?
Chleb is an inanimate masculine noun, meaning it is a thing, not a living person or animal. For these types of nouns, the accusative case (which we use for direct objects) looks exactly the same as the basic dictionary form.
What about mleko? Why doesn't it change?
Mleko is a neuter noun (it ends in -o). Just like inanimate masculine nouns, all neuter nouns keep their dictionary form when they are used as direct objects in the accusative case.
Why is the word a used for 'and' here instead of i?
In Polish, i is used simply to link things together. The word a is used to show a contrast or a shift in focus between two different subjects. Because we are contrasting what I am doing with what you are doing, a is the natural choice.
The verb forms are kupuję and kupujesz. What is the basic form of this verb?
The dictionary form (infinitive) is kupować (to buy). Verbs that end in -ować are a little tricky because that ending changes to -uj- before you add the endings for the person, like -ę for 'I' and -esz for 'you'.
Do I have to include the pronouns (I) and (you) in this sentence?