を勉強してるの

are you studying

→ を
phrase Rare #37,761
are you studying A casual question pattern meaning “are you studying [something]?” The を marks what is being studied, and してる is a casual contraction of している. The final の gives the question a conversational tone.
JA: 今、何を勉強してるの?
EN: What are you studying now?

何の教科を勉強してるの

What subject are you studying?

いつからラテン語を勉強してるの

How long have you been studying Latin?

フランス語を勉強してるの

Are you still studying French?

どんな言語を勉強してるの

What language are you studying?