Learn Hardcore Russian: В подъезде нужен порядок, поэтому нельзя бросать мусор. - There needs to be order in the stairwell, so you must not throw trash.

в
- the Prepositional case is used for location (in/at a place).
подъезд → в подъезд-е (Prepositional singular).
So it’s literally in the подъезд.

What exactly is подъезд? Is it just “entrance”?

Подъезд usually means the shared entrance/stairwell area inside an apartment building (often including the lobby, staircases, elevator area). It’s more specific than English entrance, which can mean a doorway or a way in.

How does нужен порядок work? Who “needs” what?

This is an impersonal “need” construction: (somewhere) нужен (something) = (somewhere) order is needed / required.

порядок is the thing “needed” (nominative).
нужен agrees with порядок (masculine singular).
No explicit subject like мы is required.

Why is it нужен and not нужно?

Because нужен agrees with the noun порядок (masculine singular). Compare:

Нужен порядок (masc.)
Нужна тишина (fem.)
Нужно время (neut.)
Нужны документы (plural)

Could I also say В подъезде нужен порядок vs Порядок нужен в подъезде? Does word order matter?

Both are correct. Word order mainly affects emphasis:

В подъезде нужен порядок emphasizes the place (in the entrance hall).
Порядок нужен в подъезде emphasizes порядок (order/cleanliness) as the topic.
Russian word order is flexible, but the neutral/typical version here is the one in your sentence.

What does поэтому mean, and why is there a comma before it?

поэтому means therefore / that’s why / so.
The comma appears because it links two parts of a compound sentence:
[In the entrance hall, order is needed], therefore [it’s not allowed to throw trash].

How does нельзя бросать мусор work grammatically?

нельзя is an impersonal word meaning it’s not allowed / one must not / it’s forbidden.
It is commonly followed by an infinitive: нельзя + infinitive → нельзя бросать (must not throw).

Is нельзя the same as не надо or нельзя vs не следует?

They’re related but not identical:

нельзя = not allowed / forbidden (strong, rule-based)
не надо = don’t (need to) / no need to (often softer, advice-like)
не следует = one should not (formal advice/moral recommendation)
In a building-rule context, нельзя is very natural.

Why is it бросать and not бросить?

бросать is imperfective, used for general rules and repeated/ongoing actions: “don’t throw (as a habit / in general).”
бросить (perfective) would sound more like a single completed act: “don’t throw it (once).” Rules typically use imperfective.

Is бросать мусор the most natural phrase? Could it be выбрасывать мусор?

Both exist, but the nuance differs:

бросать мусор = to drop/throw litter (often implies carelessly leaving it)
выбрасывать мусор = to throw away trash (normally into a bin)
In an entrance hall context, бросать мусор strongly suggests littering.

Why is мусор in the form мусор (not changed)?

Because мусор is the direct object of бросать, so it’s Accusative. For inanimate masculine nouns, Accusative = Nominative:

мусор (Nom) → мусор (Acc)

Could I replace порядок with something like чистота? How would the sentence change?

Yes, and the agreement changes:

В подъезде нужна чистота, поэтому нельзя бросать мусор.
Here чистота is feminine, so you use нужна instead of нужен.