Learn Hardcore Russian: Моя подруга стала журналисткой, а её мама была медсестрой. - My female friend became a journalist, and her mother was a nurse.

In Russian, after verbs like стать (to become), быть (to be, in the past/future), работать (to work as), the profession is normally put in the instrumental:

Моя подруга стала журналисткой. – My (female) friend became a journalist.
- журналистка → журналисткой (instrumental singular, feminine)
Её мама была медсестрой. – Her mother was a nurse.
- медсестра → медсестрой (instrumental singular, feminine)

So the pattern is:
стать / быть / работать кем? → instrumental case.

What exactly does the verb стала mean here, and how is it different from была?

Both verbs are in the past tense, feminine singular:

стала – past of стать, perfective, “became” (change of status / new profession)
была – past of быть, imperfective, “was” (state in the past, no focus on change)

So:

Моя подруга стала журналисткой – She wasn’t a journalist before; at some point she became one.
Её мама была медсестрой – At some time in the past, her mother was a nurse (we’re not talking about how she came to be one, just stating the fact).

You could say:

Моя подруга была журналисткой. – My friend was a journalist (at some time in the past).
This does not emphasize the moment of becoming; it just describes her past profession.

Why is it подруга and not друг for “friend”?

Russian normally marks the gender of the person:

друг – (male) friend
подруга – (female) friend

English friend is gender‑neutral, but Russian usually chooses the form according to the person’s gender.
So моя подруга tells us the friend is female.

What’s the difference between подруга and девушка? Could I say Моя девушка стала журналисткой?

Yes, but the meaning changes.

подруга – female friend (could be just a friend, not romantic)
девушка – literally “young woman / girl”, but in modern Russian it very often means girlfriend (romantic partner)

So:

Моя подруга стала журналисткой. – My female friend became a journalist.
Моя девушка стала журналисткой. – My girlfriend became a journalist.

If you just mean a non‑romantic friend, подруга is safer and clearer.

Why is it моя подруга and её мама, not мой подруга or её подруга? How do the possessive adjectives agree?

Possessive words like мой, твой, его, её, наш, ваш, их agree in gender, number, and case with the noun they describe:

подруга – feminine singular, nominative
- so: моя подруга (feminine form of мой)
мама – feminine singular, nominative
- so: её мама (same form for all genders, but agrees in case/number: feminine singular nominative here)

You’d say:

мой друг – my (male) friend
моя подруга – my (female) friend
моё письмо – my letter (neuter)
мои друзья – my friends (plural)

What does the conjunction а add here? Why а её мама была… and not и её мама была…?

Both а and и can translate as “and”, but they’re used differently:

и – neutral “and”, just adding information.
а – often contrasts or compares two things: “while / whereas / but”.

In the sentence:

Моя подруга стала журналисткой, а её мама была медсестрой.

а signals contrast/comparison between:

the daughter’s profession: journalist,
the mother’s profession: nurse.

You could translate the nuance as:

My friend became a journalist, *whereas her mother was a nurse.*

Why is the mother’s profession also in the instrumental: медсестрой after была?

Because when you say that someone is / was / will be a profession, role, or status, Russian usually uses the instrumental case with быть (especially in the past and future):

Она была медсестрой. – She was a nurse.
Он был учителем. – He was a teacher.
Он будет врачом. – He will be a doctor.

Using медсестра in the nominative (она была медсестра) is possible in some colloquial or old‑fashioned contexts, but медсестрой is the normal, standard form.

Why do both журналисткой and медсестрой end in ‑ой? Is that always the instrumental ending?

For feminine nouns in ‑а / ‑я, the usual instrumental singular ending is ‑ой or ‑ей:

журналистка → журналисткой
медсестра → медсестрой
мама → мамой
тетя → тетей

There are also alternative older/poetic forms in ‑ою / ‑ею:

журналисткою, медсестрою, мамою – correct but sound old‑fashioned or very formal/poetic.

So yes, ‑ой / ‑ей is the usual instrumental singular ending for feminine nouns in ‑а / ‑я.

Could I say Моя подруга была журналисткой, and what’s the difference from стала журналисткой?

Yes, both are grammatical but they focus on different things:

Моя подруга была журналисткой.
- Describes a state in the past: at some time, she was a journalist. We don’t say how it changed or started.
Моя подруга стала журналисткой.
- Describes a change: she became a journalist; before that, she wasn’t.

So была = was (state), стала = became (transition/change).

Why is стала used and not становилась? What’s the aspect difference?

стала – past tense of стать, perfective aspect: focuses on the result or completed change (“she became”).
становилась – past tense of становиться, imperfective aspect: focuses on the process or repeated / incomplete action (“she was becoming / used to become”).

In normal speech about getting a profession, you almost always use the perfective:

Моя подруга стала журналисткой. – She became a journalist (finished change, result).

Становилась журналисткой sounds unusual unless you’re emphasizing the ongoing process in some special context (e.g. stylistic, narrative).

Why is мама grammatically feminine even if we already know it’s a mother? How does this affect the verbs?

Мама is always a feminine noun in Russian, both grammatically and in meaning. So:

It takes feminine agreement:
- её мама была медсестрой (verb in feminine past: была)
- её мама устала – her mother got tired (feminine past form устала, not устал)

Even if context is obvious, Russian grammar still requires consistent feminine agreement with мама.

Can the word order change, for example Подруга моя стала журналисткой?

Yes, Russian word order is relatively flexible. You can say:

Моя подруга стала журналисткой. – neutral, most common.
Подруга моя стала журналисткой. – possible, but sounds more poetic, expressive, or colloquial; it can emphasize подруга (“my friend, she became a journalist”).

The basic grammar (cases, endings, agreement) doesn’t change; only the emphasis and style do. The neutral, textbook order is the original one.

What is the difference between её мама and just мама here? Do we need the pronoun её?

её мама – her mother, explicitly linked to подруга (the friend).
мама alone – could be “(my) mom” from the speaker’s perspective, or just “the mother” in a very broad context.

In this sentence, её мама clearly tells us whose mother we’re talking about: the mother of my friend.

You could also say:

Моя подруга стала журналисткой, а мама была медсестрой.

This would usually be understood as:

My friend became a journalist, and (my) mom was a nurse.

So её is necessary here to avoid confusion and to show that we are talking about the friend’s mother, not the speaker’s.