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 Она, моя сестра-студентка, говорит…?

Russian often omits personal pronouns when the subject is clear from context or from the noun itself. Here моя сестра-студентка is the subject, so adding она is unnecessary.

You could say something like:

Она, моя сестра-студентка, говорит…

but that sounds like you’re adding an afterthought or apposition (“She, my sister who is a student, says…”). In a normal neutral sentence, just using the noun phrase моя сестра-студентка as the subject is the most natural choice.

What exactly does что do in говорит, что биология сложная…? Is it the same что as “what”?

Here что is a conjunction, not a question word. It means that, introducing a subordinate clause:

Она говорит, что биология сложная…
→ She says *that biology is difficult…*

Compare:

Что ты сказал? – What did you say? (interrogative pronoun)
Она сказала, что придёт. – She said that she will come. (conjunction)

So in this sentence, что does not mean what; it links the main clause (моя сестра-студентка говорит) with the content of what she says (биология сложная, но интересная).

Could I leave out что and say Моя сестра-студентка говорит, биология сложная…?

In standard written Russian, you normally need что here. Without it, the sentence sounds incomplete or at least very colloquial.

You might hear something like:

Она говорит: биология сложная, но интересная.

Here the colon replaces что, and it sounds like direct speech (she says: “biology is difficult but interesting”).

But in the structure you gave, with a comma and indirect speech, the natural form is:

Моя сестра-студентка говорит, что биология сложная, но интересная.

So: keep что in this pattern.

Why is there no word for “is” in биология сложная?

Russian normally drops the verb “to be” in the present tense in sentences like “X is Y”.

So:

English: Biology *is difficult.*
Russian: Биология сложная. (literally “Biology difficult.”)

In the present tense, есть (the verb to be) is usually omitted in simple statements:

Он врач. – He is a doctor. (not он есть врач in normal speech)
Она умная. – She is smart.

The verb appears in other tenses and some special constructions:

Биология была сложной. – Biology was difficult.
Биология будет сложной. – Biology will be difficult.

Why do сложная and интересная both end in -ая?

They are adjectives agreeing with биология, which is:

feminine gender
singular
nominative case

The standard ending for long-form adjectives in the feminine singular nominative is -ая (or -яя after certain consonants):

сложная – difficult (fem. nom. sg.)
интересная – interesting (fem. nom. sg.)

If the noun changed, the adjective endings would change to match:

сложный предмет – a difficult subject (masc. nom. sg.)
сложное задание – a difficult task (neut. nom. sg.)
сложные задания – difficult tasks (plural)

So сложная, но интересная is exactly matching биология in gender, number, and case.

Why does the adjective come after the noun in биология сложная, instead of before it like in English?

In Russian there are two main patterns:

Attributive (adjective before noun):
- сложная биология – “difficult biology” as a description of a type or kind of biology.
Predicative (noun + adjective):
- биология сложная – “biology is difficult.”

In your sentence, биология сложная, но интересная is a statement about biology, not a compound noun phrase. So the structure is:

subject: биология
predicate: сложная, но интересная

This is the normal way to say “X is Y” with adjectives in Russian.

Why is there a comma before что and before но?

Two different rules are at work:

Comma before что
- говорит, что биология сложная…
Что introduces a subordinate clause (что биология сложная, но интересная). Russian always separates such clauses from the main clause with a comma.
Comma before но
- биология сложная, но интересная
Но is a coordinating conjunction meaning but. In Russian, a comma is normally required before но when it connects two clauses or two parts of a predicate:
- Он устал, но доволен. – He is tired but satisfied.

So both commas are mandatory according to standard punctuation rules.

How do I know that биология is feminine, and do other words ending in -ия behave the same way?

Most nouns ending in -ия are feminine, especially abstract or academic words:

биология – biology
история – history
география – geography
экономия – economy / saving
симпатия – sympathy

They decline like other -я feminine nouns. For example, биология:

Nom.: биология
Gen.: биологии (нет биологии)
Dat.: биологии (к биологии)
Acc.: биологию (любить биологию)
Instr.: биологией (интересоваться биологией)
Prep.: биологии (говорить о биологии)

So when you see -ия, assume feminine unless you have a special exception (which is rare).

Why is the verb говорит and not some other form like говорю or говорят?

Говорит is the 3rd person singular present tense of говорить (to speak, to say), and it must agree with the subject моя сестра-студентка (she).

Present-tense forms of говорить:

я говорю – I speak / I say
ты говоришь – you (sg.) speak
он / она говорит – he / she speaks
мы говорим – we speak
вы говорите – you (pl./formal) speak
они говорят – they speak

Since we are talking about она (my sister), the correct form is говорит.