Learn Croatian: U četvrtak imamo težak ispit. - We have a difficult exam on Thursday.

In Croatian, to say that something happens on a specific day of the week, you use the preposition u followed by the accusative case. So 'on Thursday' becomes u četvrtak.

Is četvrtak in the accusative case here? It looks just like the dictionary form.

Yes, it is! Because četvrtak (Thursday) is a masculine noun that doesn't represent a living being (it is 'inanimate'), its accusative form is exactly the same as its nominative (dictionary) form.

Why do težak and ispit also keep their dictionary endings?

The phrase težak ispit (a difficult exam) is the direct object of the sentence—it is the thing we have. Just like the day of the week, ispit is a masculine, non-living noun. Because of this, both the noun and the adjective attached to it stay exactly the same in the accusative case.

I've seen težak used to mean 'heavy'. Can it also mean 'difficult'?

Exactly! Težak literally means 'heavy', but it is very commonly used to mean 'difficult' or 'hard' when talking about things like exams, tasks, or problems.