Learn Russian: Я играю на гитаре.

In Russian, when you talk about playing a musical instrument, you use the preposition на (which usually means 'on'). So literally, this sentence translates to 'I play on the guitar.'

Why does the word for guitar end in -е (гитаре) instead of -а?

Because it comes after the preposition на, the noun must change its ending to match the prepositional case. For most feminine nouns that end in -а in their dictionary form, like гитара, you drop the -а and add -е to the end.

Do I use на for playing sports or games too?

No, на is specifically used for playing musical instruments. If you want to talk about playing a sport or a board game, you use the preposition в (which usually means 'in'). For example, to say 'I play tennis', you would say Я играю в теннис.

Why is the verb играю and not играть?

The dictionary form of the verb 'to play' is играть. Because the subject of the sentence is Я (I), you must change (conjugate) the verb to match. For this verb type, you drop the -ть at the end and add -ю, giving you играю.