Вечером я хочу посидеть где‑нибудь в тихом кафе.

Вечером is the instrumental case of вечер used adverbially. Russian often uses the instrumental to express time when something happens:

• утром – in the morning
• днём – in / during the day
• вечером – in the evening
• ночью – at night

So Вечером я хочу… literally is (In) the evening I want… without needing a preposition.

Forms like в вечер or в вечере are not used in this temporal sense. For “in the evening” in everyday speech, you say вечером, or more specifically сегодня вечером (“this evening”), but never в вечер.

By default, вечером in a sentence like this usually means this coming evening / tonight, especially if the context is about plans.

• Вечером я хочу посидеть… – “This evening I want to sit / hang out…”

If you wanted “in the evenings (as a habit)”, Russian typically uses по вечерам:

• По вечерам я люблю посидеть где‑нибудь в тихом кафе.
  “In the evenings I like to sit somewhere in a quiet café.”

So:

• вечером – usually one specific evening (often “this evening”)
• по вечерам – habit, repeated action in the evenings

The key difference is aspect and nuance of duration:

• сидеть – imperfective: to sit (be in a sitting state), or to be spending time sitting.
• посидеть – perfective with по‑, usually means “to sit for a while / to spend some time sitting.”

So:

• Я хочу сидеть в кафе.
  Sounds like “I want to be (in a state of) sitting in a café” – more static, not very natural if you mean “I want to go and hang out there.”

• Я хочу посидеть в кафе.
  “I want to go (there) and sit for a while / hang out in a café.”
  This sounds natural for making plans for the evening.

In this sentence, посидеть emphasizes a limited, pleasant period of time spent in a café, not just the physical posture of sitting.

The prefix по‑ often makes verbs mean “do something for a while / to some (limited) extent”.

With сидеть:

• сидеть – to sit (be sitting).
• посидеть – to sit for a while, to spend some time sitting somewhere (often implying a relaxed, temporary action).

Other similar examples:

• читать → почитать – to read → to read for a bit
• говорить → поговорить – to talk → to have a talk (for some time)
• подождать is slightly different, but ждать → подождать also often means “to wait for a bit.”

So хочу посидеть is very natural for “I feel like spending some time (chilling) there.”

In this context, посидеть в кафе does not focus on the physical action of putting your body in a sitting position. It means:

to spend some time in a café (usually sitting, relaxing, maybe drinking coffee, talking, etc.)

This is a common Russian way to express the idea of “hang out / chill / spend some time” in a place:

• посидеть в баре – to hang out in a bar for a while
• посидеть у друзей – to spend some time at friends’ place

So the sentence is more like: “In the evening I want to hang out somewhere in a quiet café.”

Both mean roughly “somewhere”, but their nuances differ:

• где‑то – “somewhere (but I don’t know / don’t say exactly where).”
  More neutral, often just “some unspecified place.”

• где‑нибудь – “somewhere / anywhere” and can sound more indefinite or carefree, sometimes “it doesn’t matter where.”

In this sentence:

• посидеть где‑нибудь в тихом кафе
  suggests “sit somewhere or other in a quiet café, any quiet café is fine.”

If you said где‑то в тихом кафе, it would still be understood, but где‑нибудь here nicely fits the relaxed, non‑specific idea: “just somewhere.”

Yes, you could say:

• Вечером я хочу посидеть в каком‑нибудь тихом кафе.

This is also natural and means almost the same thing.

Nuance:

• где‑нибудь в тихом кафе – focuses on place in general: “somewhere in a quiet café.”
• в каком‑нибудь тихом кафе – focuses on the café itself: “in some quiet café (some café or other).”

Both express that you don’t care which specific café, as long as it’s quiet. In many contexts they are interchangeable.

After в you can get different cases depending on the meaning:

• в
  • accusative → direction / into (куда?).
• в
  • prepositional → location / in (где?).

Here it’s location: где? в тихом кафе.
So тихом is prepositional case, singular, neuter:

• Masculine: тихий → в тихом доме
• Feminine: тихая → в тихой комнате
• Neuter: тихое → в тихом кафе

The noun кафе itself is indeclinable (its form doesn’t change), but the adjective тихий must agree with it in gender/number/case, so it becomes в тихом кафе.

Кафе is one of those Russian nouns that are indeclinable:

• It keeps the same form in all cases: кафе, кафе, кафе…

Its grammatical gender is neuter. You can see this from adjectives and pronouns:

• тихое кафе, моё кафе, это кафе

So in в тихом кафе:

• тихом shows the prepositional case.
• кафе stays the same but is understood as neuter, singular, prepositional.

With places like cafés, restaurants, shops, etc., Russian normally uses в to mean “in / inside”:

• в кафе – in a café
• в ресторане – in a restaurant
• в баре – in a bar
• в магазине – in a shop

На is used with some other types of locations/events:

• на работе – at work
• на концерте – at a concert
• на вечеринке – at a party
• на остановке – at the bus stop

So посидеть в кафе is the standard way to say “sit / hang out in a café.”

Yes, Russian allows quite flexible word order. These are all grammatically correct:

• Вечером я хочу посидеть где‑нибудь в тихом кафе.
• Я хочу вечером посидеть где‑нибудь в тихом кафе.
• Я вечером хочу посидеть где‑нибудь в тихом кафе.

They all mean essentially the same thing. Differences are mostly about what you emphasize or what flows better in context. Starting with Вечером puts more focus on when:

• Вечером я хочу посидеть… – As for the evening, what I want is to sit…

In everyday speech, any of these orders would sound natural.

Approximate stress and pronunciation (stressed syllables in bold):

• ВЕ́чером – VE‑che‑rom (stress on VE; ч is like English ch).
• я – ya.
• хоЧУ́ – kha‑CHU (х = like German ch in Bach; stress on chu).
• посиДЕ́ть – pa‑si‑DYET’ (soft ть at the end; stress on деть).
• где‑НИ́будь – gdye‑NEE‑boot’ (soft дь at the end, stress on ни).
• в – v (very short).
• ТИ́хом – TEE‑kham (х again like German ch; stress on ти).
• кафЕ́ – ka‑FE (stress on fe).

Natural rhythm in speech would roughly be:

ВЕ́чером я хоЧУ́ посиДЕ́ть где‑НИ́будь в ТИ́хом кафЕ́.