Learn Russian: Эта галактика, в которой мы живём, очень красивая. - This galaxy we live in is very beautiful.

In English, we often put the preposition at the end of the sentence, like 'the galaxy we live in'. In Russian, you can never end a clause with a preposition. It must go right before the word it attaches to, which is why it translates literally as 'the galaxy in which we live'.

Why is it которой and not которая?

The word for 'which' is который. Here, it refers back to галактика (galaxy), so it starts out as the feminine которая. However, because it follows the preposition в (in) to mean 'in which', it must change to the Prepositional case, becoming которой.

Why do Эта and красивая have those specific endings?

Галактика ends in -а, making it a feminine noun. Both the demonstrative pronoun ('this') and the adjective ('beautiful') must match its gender and case. Because it is the main subject of the sentence, we use the feminine Nominative forms Эта and красивая.

Why are there commas around в которой мы живём?

In Russian, relative clauses (extra information starting with words like 'which', 'who', or 'that') must always be separated from the main sentence by commas. English sometimes leaves these commas out, but Russian grammar is very strict about including them!