What's the difference between "shiteirundesuka" and "shiteimasuka"?

Question:

空いた時間に日本語を勉強しているんですか？

Why not shiteimasuka? What does that "-run-" mean?

Answer:

いるis the dictionary from of います and is the form used for this sentence ending pattern
so いるんis the shortened form of いるの
のis adding emphasis to the clause with the impression of meaning 'for the purpose of/the expectation is that'

so I think the difference between the sentences would be:
していますか-are you studying japanese in your spare time?
しているんですか-so you are studying japanese in your spare time?

Perhaps the latter emphasises that this is a conclusion made from prior information they have received