Learn Hardcore Czech: V lese za naším sousedstvím je večer tma a není tam žádné světlo. - In the forest behind our neighborhood it is dark in the evening and there is no light there.

So v lese literally means in (the) forest.
v lesu would be wrong here in standard Czech, and v les would be using the nominative, which cannot follow v in this meaning.

What case is za naším sousedstvím, and why is it not za naše sousedství?

The preposition za can take either:

Accusative – when there is motion to behind something
- e.g. jdu za dům – I am going behind the house
Instrumental – when it’s a fixed location behind something
- e.g. bydlím za domem – I live behind the house

In za naším sousedstvím we talk about a fixed place (in the forest behind our neighborhood), so:

za
- instrumental is required.
sousedství is neuter; its instrumental singular is sousedstvím.
The possessive pronoun náš must also be in the instrumental: naším.

So:

za naše sousedství (accusative) would mean to a place behind our neighborhood (motion).
za naším sousedstvím (instrumental) = behind our neighborhood (static location), which is what we need here.

Why is the pronoun naším and not naše or našem?

Naším is the instrumental singular form of the possessive pronoun náš (our) for neuter nouns.

We need:

Case: instrumental (because of za with static location)
Number: singular (one neighborhood)
Gender: neuter (sousedství is neuter)

Forms of náš in singular:

Masc. inanimate: náš / našeho / našemu / náš / našem / naším
Fem.: naše / naší / naší / naši / naší / naší
Neuter: naše / našeho / našemu / naše / našem / naším

So for neuter instrumental singular, the correct form is naším → za naším sousedstvím.

What exactly is the function of večer here? Why is there no preposition like “in the evening”?

In je večer tma, the word večer is a time adverbial (“in the evening”) made from a noun in the accusative case.

Czech very often expresses time with a bare noun in the accusative, without a preposition:

ráno – in the morning
odpoledne – in the afternoon
večer – in the evening
v noci – at night (here a preposition is used)

So je večer tma = it is dark in the evening.

You could say večer je v lese… tma (putting večer at the beginning) to emphasise the time more, but you wouldn’t use something like v večer.

Why is the word order je večer tma and not je tma večer or tma je večer?

All of these orders are grammatically possible, but they differ in emphasis (information structure), not in basic truth.

V lese… je večer tma…
= neutral, focuses on what it is like in the forest in the evening.
V lese… je tma večer…
= sounds a bit marked; it may suggest contrast (it’s dark in the evening, but maybe not at other times).
V lese… tma je večer…
= puts more emphasis on tma, almost like “dark it is in the evening” – more poetic/unusual in normal speech.

Czech word order is relatively flexible. The typical neutral pattern is to put known information first and new or important information later. Here, the location (v lese za naším sousedstvím) and time (večer) are setting the scene, and tma is the thing being said about that situation.

In je večer tma and není tam žádné světlo, which words are the subjects?

Even though the order looks different from English, the subjects are:

tma in je večer tma (“darkness is (there) in the evening”),
žádné světlo in není tam žádné světlo (“no light is (there)”).

Czech often puts the subject after the verb in these existential sentences:

Je tma. – It is dark. (lit. Darkness is.)
Je tu voda. – There is water here.
Není tam žádné světlo. – There is no light there.

So the verb být (“to be”) does not take an object here; it links to a subject that names what exists (or doesn’t exist) in that place/time.

Why do we need žádné in není tam žádné světlo? Isn’t that a double negative?

Yes, it is a double negative, and that is normal and required in standard Czech.

In negative sentences about existence/quantity, Czech uses:

a negative verb (není, nemá, nevidím…)
plus a negative pronoun/determiner (žádný, nikdo, nic,…).

Examples:

Není tam žádné světlo. – There is no light there.
Nemám žádné peníze. – I have no money.
Nevidím nikoho. – I see nobody.

If you said Není tam světlo without žádné, it would sound strange or incomplete, almost like “The light isn’t there” (referring to some specific light previously known), not the general idea “there is no light at all.”

Why is it žádné světlo and not žádný světlo or žádná světla?

Because žádné must agree with the noun světlo in:

gender: neuter
number: singular
case: nominative (subject of není)

The noun světlo (light) is neuter:

Singular nominative: světlo
Plural nominative: světla

The matching forms of žádný are:

Masc. animate: žádný
Masc. inanimate: žádný
Feminine: žádná
Neuter singular: žádné
Neuter plural: žádná (e.g. žádná světla = no lights)

So:

žádné světlo = no light (at all, as a substance/phenomenon)
žádná světla = no lights (no individual light sources, lamps, etc.)

Both can be correct in principle, but the original sentence talks about the general absence of light, so žádné světlo is the natural choice.

What is the role of tam in není tam žádné světlo, and could we leave it out?

Tam means there and points back to the location already mentioned (the forest behind our neighborhood).

Není tam žádné světlo. = There is no light there.

You could grammatically say:

Není žádné světlo. – There is no light. (general statement, no specific place)
Or you could move tam: Tam není žádné světlo. – There is no light there. (slightly stronger emphasis on there)

In the first clause, the location is expressed explicitly: V lese za naším sousedstvím, so adding tam there would be redundant:
V lese za naším sousedstvím je večer tma is enough.

In the second clause, tam smoothly refers back to that location and keeps the sentence natural and clear.

What is the difference between tma and světlo here? Are they adjectives or nouns?

Both tma and světlo are nouns:

tma – darkness (feminine noun)
světlo – light (neuter noun)

They are used as subjects of the verb být:

Je tma. – It is dark. (lit. Darkness is.)
Není žádné světlo. – There is no light.

In English, we usually use adjectives (dark) with a dummy subject (it), but Czech often uses concrete nouns for abstract states like this.

Czech adjectives would be:

tmavý – dark (describing something: tmavý les = a dark forest)
světlý – light/bright (describing something: světlý pokoj = a bright room)

In this sentence, we are not describing a specific object (e.g. a dark forest as a property), but rather stating what the situation is like: there is darkness, there is no light.

Could we use bývá instead of je: V lese… večer bývá tma? What would change?

Yes, you can say:

V lese za naším sousedstvím bývá večer tma a není tam žádné světlo.

The difference:

je – a simple statement of fact right now (or generally true without stressing regularity).
bývá – habitual/typical: it tends to be / it is usually / it is often.

So:

…je večer tma… – it is dark there in the evening.
…bývá večer tma… – it (usually) gets / is dark there in the evening (as a regular occurrence).

Is it possible to drop je in je večer tma, like in Russian, and just say večer tma?

In standard Czech, you normally must keep the present tense of být (je) in such sentences. Unlike Russian, Czech does not systematically drop the present-tense copula.

Correct: Večer je tma.
Very colloquial/telegraphic (headlines, slogans): Večer – tma. (but this is stylistic, not neutral grammar)

For regular spoken and written Czech, you should say je večer tma, not just večer tma, if you want a full, normal sentence.