Learn French: Est-ce que tu prends des leçons de violon depuis un an ? - Have you been taking violin lessons for a year?

In French, if an action started in the past and is still continuing right now, you use the present tense. English uses a complicated past tense ("have been taking"), but French keeps it simple: you use the present tense (tu prends) plus depuis to show the action is still happening.

The English sentence says "for a year". Why do we use depuis instead of pour?

Depuis translates to both "since" (a starting point, like 2015) and "for" (an amount of time, like a year) when referring to ongoing actions. You should avoid using pour to mean "for" with time, unless you are talking about a scheduled duration in the future (like going on vacation pour une semaine).

Why is it leçons de violon without an article like du or le before violon?

When you use a noun to describe the type or category of another noun (a "violin lesson", a "coffee cup"), you connect them with a simple de. You aren't talking about a specific violin, so you don't use du (which is de + le).

Why do we use an here instead of année for "year"?