Learn Croatian: Liječnik sluša tvoja pluća. - The doctor is listening to your lungs.

The noun pluća (lungs) is grammatically a neuter plural noun in Croatian. Because adjectives must agree with the noun's gender and number, the possessive adjective takes the neuter plural ending -a, becoming tvoja.

What case is tvoja pluća in, and why doesn't it change its ending?

It is in the accusative case because it's the direct object of the verb sluša. It doesn't change its ending because for all neuter nouns, the nominative and accusative forms are completely identical in both singular and plural.

Why is there no preposition for the English word "to" in this sentence?

Unlike the English phrase "listen to", the Croatian verb slušati takes a direct object without needing any preposition. You literally say the doctor "listens your lungs".

Is there a difference between liječnik and doktor?

They are used interchangeably for medical professionals, but there is a slight difference in tone. Liječnik is the native Croatian word (from the verb liječiti, meaning to treat or heal) and is preferred in formal contexts. Doktor is an international word used heavily in everyday speech, but it can also refer to anyone with an academic doctorate.