Learn Croatian: Uvijek uživam u slaganju velike slike. - I always enjoy assembling a big picture.

The verb uživati (to enjoy) requires the preposition u followed by the locative case when you are talking about enjoying an activity or a thing. In Croatian, you literally say "I enjoy in assembling."

How is the word "slaganju" formed?

It is the locative form of slaganje, which is a verbal noun meaning "assembling" or "putting together." Verbal nouns are formed from imperfective verbs (in this case, slagati) and decline just like regular neuter nouns ending in -je.

If the verb "slagati" takes the accusative case, why are "velike slike" in the genitive case here?

This is a crucial rule to remember. When a verb turns into a verbal noun, its direct object changes from the accusative to the genitive case. Instead of saying "assembling a big picture," Croatian grammar requires "the assembling OF a big picture," which is why we use the genitive velike slike.

Could I use an infinitive verb instead, like "uživam slagati"?

No, standard Croatian does not allow uživati to be followed directly by an infinitive. You must either use u + the locative of a verbal noun (as in this sentence) or rephrase it with a clause, such as uživam dok slažem (I enjoy while I assemble).