Learn Hardcore Russian: Моя подруга расстроилась вечером. - My female friend got upset in the evening.

In many neutral contexts, моя подруга = “my (female) friend.”
For a romantic girlfriend, people also often say:
- моя девушка – my girlfriend (most common)
- моя женщина – my woman (adult, more serious/colloquial, can sound quite strong)

So in this sentence, моя подруга расстроилась вечером, the default reading is “my (female) friend got upset in the evening,” unless context has clearly marked it as romantic.

Why is it моя подруга (nominative) and not мою подругу (accusative)?

Моя подруга is the subject of the sentence – the person who performs/experiences the action расстроилась. Subjects in Russian normally stand in the nominative case.

Моя подруга (nom.) – subject: “My (female) friend”
расстроилась – verb: “got upset”

If you used мою подругу (accusative), that would imply she is an object of someone else’s action, e.g.:

Я расстроил мою подругу. – I upset my (female) friend.

In your sentence, she is not being affected by someone else grammatically; she herself “got upset,” so nominative is correct: моя подруга.

What is the infinitive of расстроилась, and what does the -сь / -ся ending mean?

The infinitive is расстроиться.

расстроить – to upset (someone)
расстроиться – to get upset / to become upset (reflexive / intransitive)

The ending -сь (or -ся, depending on the preceding sound) marks the verb as reflexive. Here it means the action happens to the subject itself, not to someone else.

Compare:

Я расстроил подругу. – I upset my (female) friend.
Подруга расстроилась. – My (female) friend got upset (herself / her mood changed).

Why does the verb end in -лась (расстроилась)?

In Russian, past tense verbs agree with the subject in gender and number.

The pattern for many past tense verbs is:

Masculine singular: -лся (or just -л if not reflexive)
Feminine singular: -лась
Neuter singular: -лось
Plural (for all genders): -лись

Since подруга is feminine singular, the verb takes the feminine singular past form:

Она расстроилась. – She got upset.
Он расстроился. – He got upset.
Они расстроились. – They got upset.

So расстроилась matches моя подруга in gender and number.

Why is the perfective расстроилась used instead of расстраивалась?

расстроилась is perfective; расстраивалась is imperfective.

расстроиться (perf.) – focuses on the result / change of state: “(to) become upset,” “(to) get upset.”
расстраиваться (impf.) – focuses on the process, repeated or ongoing state: “to be getting upset,” “to get upset (repeatedly / habitually),” “to be upset (over a period).”

In Моя подруга расстроилась вечером, the idea is that at some point in the evening, she became upset; it’s a single completed change of state – that’s why perfective fits naturally.

If you said:

Моя подруга расстраивалась вечером.

it would tend to mean something like “She was upset in the evening” / “She would get upset in the evenings” (process or repeated situation), depending on context.

Does расстроилась mean “got upset” (change) or “was upset” (state)?

Because расстроилась is perfective, it primarily expresses a change of state: “got upset,” “became upset.”

To emphasize a state (being upset for some time), Russian more often uses the imperfective or a different construction, for example:

Она была расстроена вечером. – She was upset in the evening.
Она расстраивалась вечером. – She was (getting) upset in the evening / She used to get upset in the evenings.

So in your original sentence, the main nuance is that at some point during the evening, her mood changed and she became upset.

Why is there no pronoun она in the sentence? Would Она расстроилась вечером also be correct?

Russian often omits subject pronouns when the subject is already clear from context, from a noun, or from the verb ending.

In Моя подруга расстроилась вечером:

Моя подруга is the explicit subject, so она is not needed.

You could say:

Она расстроилась вечером. – She got upset in the evening.

This is also correct, but now the subject is the pronoun она, not моя подруга. You would usually use она if the person is already known from context and you don’t want to repeat their name or description.

A bit redundant would be:

Моя подруга, она расстроилась вечером. – “My friend, she got upset in the evening.” (possible, but stylistically heavier, like an afterthought or emphasis).

Why is it вечером with -ом and no preposition, instead of something like в вечер?

Вечером is the instrumental case of вечер (“evening”) used adverbially to mean “in the evening / during the evening.” No preposition is needed.

Russian often uses the instrumental case of time words to mean “at/in that part of the day”:

утром – in the morning
днём – in/at the daytime
вечером – in the evening
ночью – at night (here it’s an adverbial form, not instrumental in form, but similar function)

Saying в вечер is not correct in standard Russian. You can say:

в этот вечер – on that evening
в тот вечер – that evening

but not bare в вечер with the general meaning “in the evening.” For that, use вечером.

Can I move вечером to the beginning or middle of the sentence? For example, Вечером моя подруга расстроилась?

Yes. Russian word order is flexible, especially for adverbs of time. All of these are grammatically correct:

Моя подруга расстроилась вечером.
Вечером моя подруга расстроилась.
Моя подруга вечером расстроилась.

The basic meaning “My (female) friend got upset in the evening” stays the same, but the focus can shift slightly:

Вечером моя подруга расстроилась. – puts a bit more emphasis on when it happened (in the evening).
Моя подруга расстроилась вечером. – more neutral; typical “subject – verb – time” structure.

In everyday speech, all variants are fine; intonation will also influence what is emphasized.

What is the difference between вечером, по вечерам, and к вечеру?

They all involve the idea of “evening,” but with different nuances:

вечером – “in the evening” (one specific evening, or contextually understood):
- Моя подруга расстроилась вечером. – She got upset in the evening (on that evening / that day’s evening).
по вечерам – “in the evenings,” regularly / habitually:
- Моя подруга по вечерам расстраивалась. – My friend would get upset in the evenings / used to be upset in the evenings.
к вечеру – “by evening,” “towards evening”:
- К вечеру моя подруга расстроилась. – By evening / toward evening, my friend had become upset.

So your original вечером expresses a time within the evening, not habitual repetition and not “by/toward evening.”

Is the sentence Моя подруга расстроилась вечером formal, informal, or neutral in style?

This sentence is neutral in style. It can be used in:

everyday spoken Russian, and
written Russian (stories, letters, reports, etc.)

There is nothing slangy or overly formal about моя подруга, расстроилась, or вечером here. It’s standard, neutral Russian.