Learn Croatian: Jabuka na tržnici je jeftina. - The apple at the market is cheap.

Because the apple is already at the market (location), not moving to the market (destination). When na shows a static location, it uses the locative case (giving feminine nouns an -i ending). If you were going to the market, you would use the accusative case: Idem na tržnicu.

Why do we use na (on) for the market instead of u (in)?

In Croatian, open-air places, public squares, and events usually take na instead of u. Because a tržnica is traditionally an outdoor farmers' market, it pairs with na.

Why is the word for cheap jeftina instead of just jeftin?

Adjectives have to match the gender of the noun they describe. Jabuka (apple) is a feminine noun, which you can guess because it ends in -a. So, the base adjective jeftin gets the feminine -a ending to become jeftina.

There is no word for "the" in the Croatian sentence. How do we know it means "The apple"?

Croatian doesn't use articles like "a", "an", or "the". The word jabuka can mean "an apple" or "the apple" depending on the situation. You just have to guess from the context! Here, because we are describing a specific apple at the market, "The apple" makes the most sense in English.