Learn Croatian: Ponekad spavaš ovdje. - Sometimes you sleep here.

The dictionary form of the verb is spavati (to sleep). Because it is a regular -ati verb, we simply drop the -ti to get the base spava-. Then, we add the ending -š to show that "you" (singular) are doing the action.

Where is the word for "you" (ti) in this sentence?

In Croatian, you usually drop subject pronouns like ja (I) or ti (you) because the verb ending already tells us who is doing the action. The -š on the end of spavaš makes it crystal clear that "you" are sleeping. You would only add ti for emphasis, as in "Ti ponekad spavaš ovdje" (It is you who sometimes sleeps here).

Is ponekad related to the word kad (when)?

Yes! The word kad or kada means "when", and nekad means "sometime". Adding the prefix po- gives us ponekad, which specifically means "sometimes" or "occasionally".

I have also learned the word tu. Can I use it instead of ovdje?

Yes, they are very similar and often used interchangeably. Strictly speaking, ovdje means right here (close to the speaker), while tu means "there" or "around here" (close to the person you are talking to). In everyday speech, however, you will frequently hear both used to mean "here".