Learn Croatian: Liječnik radi u bolnici. - The doctor works in the hospital.

The base word for hospital is bolnica. Because the doctor is in the hospital (a fixed location), we use the locative case. For feminine nouns that end in -a, we make the locative by dropping the -a and adding -i.

Does the preposition u always make the following noun end in -i?

Not always! The preposition u (in/at) forces the locative case here because it describes a fixed location (where the doctor is working). However, only feminine nouns ending in -a take the -i ending in the locative. Masculine and neuter nouns take a different ending, usually -u.

What is the dictionary form of the verb radi?

The dictionary form (infinitive) is raditi, which means to work or to do. Since liječnik (the doctor) is a "he" (third-person singular), we drop the -ti from the infinitive to get the present tense form radi.

Is the word liječnik related to medicine?

Yes! The root word lijek means medicine or drug. So a liječnik is literally a person who heals or gives medicine. You will also hear the borrowed word doktor used very commonly in everyday Croatian.