Learn Polish: Czy szybko się uczysz?

The reflexive word się is flexible, but it generally tries to hide in the middle of a sentence. It strongly prefers not to be the very first or very last word. Since the verb uczysz is at the end here, się tucks in right before it.

Why do we need the word się here at all?

In Polish, the base verb uczyć actually means 'to teach'. To say 'to learn', you must use the reflexive form uczyć się, which literally translates as 'to teach oneself'.

Where is the word for 'you' in this sentence?

Polish usually drops subject pronouns like ty (you) because the verb itself already gives the subject away. The -sz ending on uczysz tells us exactly who is doing the learning.

What exactly does czy do at the beginning of the sentence?

Czy is a special question word used to start yes/no questions. It does not have a direct translation here, but it works a lot like putting 'Do' at the start of an English question to let the listener know a question is coming.