Breakdown of Мы покупаем этот компьютер, потому что он дешёвый.
он
he / it (for a masculine noun) / it (for a neuter noun)
мы
we
компьютер
computer
этот
this [f: эта, n: это]
Questions & Answers about Мы покупаем этот компьютер, потому что он дешёвый.
Why is there a comma before потому что?
In Russian, you always put a comma before conjunctions like потому что (because), что (that), or но (but) when they connect two halves of a sentence.
Can I use почему instead of потому что here?
No. Почему is a question word meaning "why" (as in, "Why are you buying this?"). Потому что means "because" and is used to give the reason. They cannot be swapped! Also, notice that потому что is always written as two separate words.
Why does the sentence use он (he) to refer to a computer?
In Russian, all nouns have a gender. Компьютер ends in a consonant, which makes it a masculine noun. Therefore, you must use the masculine pronoun он to say "it" when referring back to the computer.
We are doing an action to "this computer", so why didn't the words этот компьютер change their endings?
You are correct that they are the direct object and are in the accusative case! However, for masculine inanimate objects (non-living things like a computer), the accusative case looks exactly the same as the dictionary form. So, the endings do not change here.
Does покупаем mean "we buy" or "we are buying"?