Learn Hardcore Russian: Вечером волонтёр пишет пост о животных зоопарка и просит людей не бросать мусор. - In the evening the volunteer writes a post about the animals in the zoo and asks people not to throw trash.

So вечером is the instrumental form of вечер, and here it functions like an adverb: вечером = in the evening. You must not say в вечером in this meaning; that would be incorrect.

If you want a more specific time, you can use в + accusative:

в этот вечер – on this evening
в пятницу вечером – on Friday evening

What aspect and tense is пишет? Does it mean “writes” or “is writing”?

Пишет is:

3rd person singular
present tense
imperfective aspect
from the verb писать (to write)

Imperfective present in Russian can correspond to both:

English “is writing” (right now)
English “writes” (regularly / habitually)

In this sentence with вечером, it likely describes a regular action:

Вечером волонтёр пишет пост…
→ In the evening the volunteer (usually) writes a post…

If you wanted a completed, one-time action in the past, you would use perfective, e.g.:

Вечером волонтёр написал пост… – In the evening the volunteer wrote (and finished) a post.

What does волонтёр mean exactly, and is there another word for “volunteer”?

Волонтёр means “volunteer” (in the modern, often NGO / project / social-activity sense). It’s masculine, stressed on -ёр: волонтёр.

Another common, more “native” Russian word is:

доброволец – literally “someone who does something voluntarily”

Nuances:

волонтёр – very common in modern Russian, especially for:
- charity work
- animal shelters
- events, festivals, sports, etc.
доброволец – can sound a bit more formal or old-fashioned, and also occurs in historical/military contexts (volunteer soldier).

In this sentence, волонтёр is perfectly natural and modern.

Why is it пост, and does this word decline like a normal Russian noun?

Пост here is a borrowing (from English post in the internet sense).

Grammatically, it behaves like a regular inanimate masculine noun:

Nom. sg: пост
Gen. sg: поста
Dat. sg: посту
Acc. sg: пост
Instr. sg: постом
Prep. sg: (о) посте

In the sentence, пишет пост is accusative singular, same form as nominative, because it’s an inanimate masculine noun.

Don’t confuse it with the other Russian пост meaning “fasting” (religious fast) or “post” as in “guard post”; they are homonyms but context makes it clear.

Why is it о животных зоопарка and not о животных зоопарке?

Two different structures, with different nuances:

о животных зоопарка
- животных – prepositional plural of животное after о
- зоопарка – genitive singular of зоопарк
Literally: about the animals of the zoo (the zoo’s animals, animals belonging to the zoo).
о животных в зоопарке
- о животных – about the animals
- в зоопарке – in the zoo (location)
Literally: about the animals in the zoo (animals that are located there).

In practice, both can overlap in meaning, but:

о животных зоопарка sounds a bit more like “the zoo’s animals as a group / as an institution’s animals.”
о животных в зоопарке emphasizes where they are (their physical location).

Why does о животных look like genitive plural? Isn’t о supposed to take the prepositional case?

Yes, о (about) normally takes the prepositional case. The confusion comes from the fact that for животное the prepositional plural and genitive plural look the same:

Nom. pl: животные
Gen. pl: животных
Prep. pl (after о): о животных

So о животных is prepositional plural, even though its form coincides with genitive plural. This happens with many neuter nouns ending in -о / -е and some others:

о зданиях – about buildings
о людях – about people (irregular)

So the rule still holds: о + prepositional, but forms can overlap with genitive.

What case is зоопарка, and why is it used here?

Зоопарка is genitive singular of зоопарк.

Here it’s used in a noun–noun construction:

животные зоопарка – the animals of the zoo

This is the same pattern as:

книга студента – the student’s book / book of the student
жители города – the inhabitants of the city

So о животных зоопарка = “about the animals of the zoo” (a genitive-of-possessor idea).

Why is there no comma before и просит людей?

Because we have one subject with two homogeneous predicates:

Subject: волонтёр
Predicate 1: пишет пост…
Predicate 2: просит людей…

In Russian, when two (or more) verbs share the same subject and are joined by и (and), there is no comma:

Он взял книгу и начал читать. – He took the book and started reading.
Волонтёр пишет пост и просит людей…

You would normally add a comma if the clauses had different subjects or if the structure were more complex (subordinate clauses, etc.), but here it’s a simple coordination of two actions by the same subject.

What is the structure просит людей не бросать мусор? Why is людей in this form?

The pattern is:

просить + accusative person + infinitive

Meaning: to ask someone to do/not do something.

просит людей – asks people (accusative plural of люди)
не бросать – not to throw (infinitive, imperfective)
мусор – trash (object of the infinitive)

For animate nouns in the plural, accusative = genitive in form:

Nom. pl: люди – people
Gen. pl: людей – (of) people
Acc. pl: людей – (ask) people

So grammatically it’s accusative plural, even though it looks like genitive.

Why is the infinitive бросать, not бросить, after не? What’s the aspect difference here?

Бросать – imperfective
Бросить – perfective

With prohibitions / requests not to do something that refer to a general, repeated, or habitual action, Russian usually uses the imperfective infinitive:

Не бросать мусор. – Don’t throw trash (in general / as a rule).
Не курить. – No smoking.
Не шуметь. – Don’t make noise.

Perfective не бросить мусор would sound like “not to throw (the) trash” as a single, specific action, and is much less natural in slogans, rules, or general requests.

So не бросать мусор = “not to litter” in a general sense.

Why is мусор singular here? Can it have a plural?

Мусор is a mass noun in Russian, like “trash / rubbish” in English, and is usually used in the singular:

мусор – trash, garbage (as a substance / mass)

It declines in the singular:

Nom: мусор
Gen: мусора
Dat: мусору
Acc: мусор
Instr: мусором
Prep: (о) мусоре

There is a plural мусоры, but it’s rare and usually means:

types of trash, or
metaphorical / specialized uses.

For ordinary “trash, litter” you use singular, so не бросать мусор is the normal, idiomatic form.

Could the word order be Волонтёр вечером пишет пост… instead of Вечером волонтёр пишет пост…? Does it change the meaning?

Both orders are grammatically correct; the difference is nuance and emphasis.

Вечером волонтёр пишет пост…
- Starts with the time frame.
- Feels like you’re setting a scene: In the evening, the volunteer writes a post…
- Time is the “starting point” of the information.
Волонтёр вечером пишет пост…
- Starts with волонтёр (the subject).
- Emphasis begins with who does the action; вечером is secondary, specifying when.

Russian word order is relatively flexible; the main function of changes is information structure (what you present as given/known vs. new/important). The basic meaning is the same; only the focus and rhythm shift slightly.

How would this sentence change if the volunteer did this every evening as a regular habit, and we wanted to stress that?

The sentence already can imply a habitual action because of the imperfective пишет and the time adverbial вечером.

If you want to make the habitual sense very explicit, you could add words like:

каждый вечер – every evening
обычно вечером – usually in the evening

For example:

Каждый вечер волонтёр пишет пост о животных зоопарка и просит людей не бросать мусор.
– Every evening the volunteer writes a post about the animals of the zoo and asks people not to throw trash.

Grammatically it’s the same structure; you only strengthen the idea of routine with adverbs like каждый, обычно, часто.