Learn Polish: Martwię się o ciebie.

The verb martwić się (to worry) is a reflexive verb, meaning it naturally pairs with się. If you drop it and just say martwię, it means you are actively worrying or troubling someone else, rather than feeling worried yourself.

Why is the preposition o used here?

In Polish, the verb martwić się pairs specifically with the preposition o to mean "to worry about." When used in this context, o forces the following word or pronoun into the accusative case.

Both ciebie and cię mean "you" in the accusative case. Why must we use ciebie here?

Polish has long and short forms for some pronouns. You must always use the long, stressed form (like ciebie) when it comes immediately after a preposition. Because it follows o, it has to be o ciebie.