Learn Hardcore Latin: Medicus iecur curat. - The doctor cares for the liver.

Medicus is in the nominative singular, the case typically used for the subject.
Iecur is in the accusative singular, the case typically used for the direct object.
The verb curat is singular, so it matches medicus: the doctor treats.

So the basic structure is:

Why doesn’t iecur end in -m if it is the direct object?

Because in Latin, not every accusative singular ends in -m.

English-speaking learners often first see examples like:

so it is easy to think direct objects always have -m. But that is only true for certain declensions and noun types.

Iecur is a neuter noun, and neuter nouns have an important rule:

So the form iecur can be both nominative and accusative singular. In this sentence, it is accusative because it is the thing being treated.

What declension is ?