Breakdown of Мой брат тоже идёт на почту.
брат
brother
тоже
also
мой
my / mine
на
on, at / by (transport) / to (a place) / to (in answer to)
Questions & Answers about Мой брат тоже идёт на почту.
Why do we use на instead of в for 'to the post office'?
In Russian, certain locations conventionally take the preposition на (on/to) instead of в (in/to). The word почта (post office) is one of these common exceptions, so we use на почту when talking about it as a destination.
Why does the word почта change to почту here?
Because it is the destination of an action! When answering the question 'to where?' (куда), Russian uses the Accusative case. For feminine nouns that end in -а like почта, the ending changes to -у.
The English translation just says 'is going'. Why use the specific verb идёт?
The verb идёт specifically means going somewhere on foot, in one direction right now. If the brother were driving or taking a bus to the post office, you would need a transport verb, like едет.
Could I put тоже at the end of the sentence, like we do with 'too' in English?
You could, because Russian word order is very flexible. However, placing тоже right before the verb (as in тоже идёт) is extremely common and usually sounds the most natural in Russian.