Learn Spanish (Spain): ¿Estudias español?

Because it follows the rule for regular -ar verbs. To say what "you" (informal) do in the present tense, you drop the -ar from estudiar and add -as. This gives us estudias (you study).

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

In Spanish, you can usually leave out subject pronouns like tú (you). Because the verb estudias ends in -as, we already know exactly who is doing the action. It can only mean "you study".

How do I say the "Do" part of "Do you study?"

Spanish doesn't use helper words like "do" or "does" to ask questions. You just take the basic statement Estudias español (You study Spanish) and add question marks and a rising tone of voice to make it a question.

Why isn't the word español capitalized?

Unlike in English, Spanish does not capitalize the names of languages or nationalities. It is always written with a lowercase letter unless it is the very first word of a sentence.