Learn Hardcore Russian: Моя сестра-студентка говорит, что биология сложная, но интересная. - My sister, a female student, says that biology is difficult but interesting.

The hyphen turns сестра-студентка into a kind of compound noun: “a sister who is a student” as one combined idea. It feels like saying my sister the student in English – it presents “student” as her defining role.

Without the hyphen:

Моя сестра студентка. – a normal sentence meaning “My sister is a student.”
Моя сестра-студентка – more like a description of which sister we’re talking about: “my sister who is a student” (for example, as opposed to another sister with a different job).

Both are grammatically correct; the hyphen just slightly changes the nuance and makes the phrase feel more compact and descriptive.

Is сестра-студентка in the nominative case? Do both words change case together?

Yes. In Моя сестра-студентка говорит…, the whole phrase моя сестра-студентка is the subject of the sentence, so it is in the nominative case.

In such hyphenated pairs, both nouns take the same case and usually decline together. For example:

Nominative (subject): моя сестра-студентка
Genitive: нет моей сестры-студентки
Prepositional: я думаю о моей сестре-студентке

So you treat the whole thing as one unit, but you still apply the normal case endings to both parts.

Why is there no pronoun она before говорит? Could I say ?