Learn Croatian: Što radiš ovdje? - What are you doing here?

The dictionary form is raditi (to do, to work). Since this is a regular -iti verb, we drop the -iti to get the stem rad-, and then add the present tense ending for singular you, which is -iš. That gives us radiš.

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

In Croatian, the present tense verb covers both simple and continuous English tenses, so radiš means both "you do" and "you are doing". Also, because the -š ending already tells us the subject is "you", we don't need to add the pronoun ti unless we want to add special emphasis.

Can radiš also mean "you work"?

Yes! The verb raditi means both "to do" and "to work". Depending on the context, Što radiš? can mean "What are you doing?" or "What is your job?". Here, the word ovdje (here) makes "What are you doing here?" the most natural meaning.

I sometimes hear people say šta instead of što. Are they the same?

Yes, they mean exactly the same thing. Što is the standard Croatian word for "what", but in everyday spoken language, you will very frequently hear šta. It is perfectly fine to use either, but you should be able to recognize both!