Breakdown of Я покупаю конверт на почте.
я
I
на
on, at / by (transport) / to (a place) / to (in answer to)
покупать
to buy
почта
post office
Questions & Answers about Я покупаю конверт на почте.
Why doesn't the word конверт change its ending? Isn't it the direct object?
It is the direct object, which means it is in the accusative case. However, because конверт (envelope) is a masculine inanimate noun (a non-living thing), its accusative form looks exactly the same as its dictionary form.
How can I tell if Я покупаю means "I buy" or "I am buying"?
Russian doesn't have a separate "am doing" tense like English does. The present tense verb покупаю covers both! Depending on the context, it translates to "I buy" (regularly) or "I am buying" (right now).
Why does this sentence use на instead of в for the post office?
Certain places in Russian traditionally take на (on/at) instead of в (in) to say where you are. Почта (post office) is one of these common exceptions. You just have to memorize на почте as a set phrase.
Why does the sentence end with почте instead of the dictionary word почта?
Because we are talking about where something is happening and using the preposition на, the noun must be in the prepositional case. For most feminine nouns ending in -а like почта, this means changing the final -а to an -е.