Learn Hardcore Czech: V našem sousedství je večer dlouho světlo, protože okna domů jsou velká. - In our neighborhood it stays light for a long time in the evening because the windows of the houses are big.

In your sentence, je večer dlouho světlo is literally “is in the evening long light”, which Czech speakers understand as “it stays light for a long time in the evening.” The “it” is just not expressed at all.

Why is it světlo and not an adjective like světlé in this sentence?

Here světlo is a noun, meaning “light” (the opposite of darkness), not an adjective meaning “light / bright.”

Je světlo. – It is light (there is light, not dark).
Je tma. – It is dark (there is darkness).

If you said:

Je světlé.

you would be using an adjective, but there is no clear noun for it to describe, so the sentence would be incomplete or strange.

You can use světlý/světlé as an adjective with a noun:

Pokoj je světlý. – The room is bright.
Je to světlý byt. – It is a bright (well‑lit) flat.

So:

Je světlo = statement about light vs dark in general.
Je světlý pokoj = statement about the quality of the room (bright).

Why is dlouho used and not dlouhé in je večer dlouho světlo?

Dlouho is an adverb, and it describes how long something lasts:

dlouho = for a long time

Czech uses the adverb here because we are describing the duration of the state “it is light”:

Je dlouho světlo. – It is light for a long time.

Dlouhý / dlouhé / dlouhá are adjectives and must describe a noun:

dlouhý den – a long day
dlouhá ulice – a long street
dlouhé okno – a long window

If you said:

Je večer dlouhé světlo,
this would sound wrong, because dlouhé is looking for a noun to modify, but here we want to modify the verb (“is”) – so we need the adverb dlouho.

How does večer mean “in the evening” without a preposition?

Czech often uses bare time nouns as adverbials of time without prepositions. So:

večer – in the evening
ráno – in the morning
odpoledne – in the afternoon
v noci – at night (this one does keep v, idiomatically)

Examples:

Večer čtu knihu. – I read a book in the evening.
Ráno pracuju. – I work in the morning.

So je večer dlouho světlo literally has večer as “when?”:

Kdy je dlouho světlo? – Večer.
When is it light for a long time? – In the evening.

Why is it v našem sousedství and not naše sousedství?

The preposition v (in) requires the locative case after it. That’s why both the adjective and the noun change form:

Nominative (dictionary form):
naše sousedství – our neighborhood (subject)
Locative after v:
v našem sousedství – in our neighborhood

Breakdown:

v – preposition “in”
našem – locative singular of náš (our), matching sousedství
sousedství – neuter noun “neighborhood,” in the locative here (it looks the same as nominative, but the function is different)

So v + našem sousedství is “in our neighborhood” with the correct case required by the preposition.

What exactly does okna domů mean, and why is it domů and not domy?

Okna domů literally means “windows of (the) houses.”

okna – nominative plural of okno (window), subject of the verb
domů – genitive plural of dům (house), used to express possession / belonging

So the structure is:

okna domů = the windows of the houses

Why not domy?

domy is nominative plural (“houses” as a subject or standalone object).
Here, domů is dependent on another noun (okna) and tells us whose windows they are, so Czech uses the genitive.

Be aware that domů is also the common adverb “home(wards)”:

Jdu domů. – I’m going home.

In your sentence, context makes it clear that domů is “of houses” (genitive plural), not the direction “home.”

Why is it okna domů jsou velká and not jsou velké?

The adjective must agree in gender, number, and case with the noun it describes.

okno = neuter singular
okna = neuter plural (subject of the sentence)

The adjective velký (big) has different forms:

masc. animate pl.: velcí
masc. inanimate pl.: velké
feminine pl.: velké
neuter pl.: velká

Since okna are neuter plural, the correct form is:

Okna … jsou velká.

Examples for comparison:

Velké domy jsou nové. – The big houses are new. (domy = masc. inanimate pl. → velké)
Velká okna jsou drahá. – The big windows are expensive. (okna = neuter pl. → velká)

Can the word order be changed? For example, can I say Večer je v našem sousedství dlouho světlo?

Yes, Czech word order is fairly flexible, especially for adverbials (time, place, manner). All of these are grammatically correct:

V našem sousedství je večer dlouho světlo.
Večer je v našem sousedství dlouho světlo.
Večer je dlouho světlo v našem sousedství.
Je večer dlouho světlo v našem sousedství.

The differences are mainly in emphasis and style:

Putting v našem sousedství first emphasizes location: In our neighborhood, ...
Putting večer first emphasizes time: In the evening, ...
Neutral / typical informative order is often:
[Time] – [Place] – [Verb + rest], e.g.
Večer je v našem sousedství dlouho světlo.

For a learner, all of the above are acceptable, but starting with V našem sousedství or Večer sounds very natural.

Why is there a comma before protože in ..., protože okna domů jsou velká?

In Czech, a comma is normally required before most subordinating conjunctions, including protože (because), when they introduce a subordinate clause.

So you write:

V našem sousedství je večer dlouho světlo, protože okna domů jsou velká.

Structure:

Main clause: V našem sousedství je večer dlouho světlo.
Subordinate clause: protože okna domů jsou velká.

This is a general punctuation rule; even if in English you sometimes omit the comma, in Czech you almost always keep it before protože (unless the sentence is very short and tightly bound, and even then the comma is usually preferred).

Why is světlo singular when there are many windows? Would světla make sense?

Here, světlo is used as an uncountable mass noun meaning “light” (as a condition: not dark). It does not refer to individual lamps or light sources.

So:

Je večer dlouho světlo. – It is light (not dark) for a long time in the evening.

The plural světla usually means:

multiple light sources / lamps
e.g. Pouliční světla – street lights
sometimes multiple distinct lights you can count

If you said:

Je večer dlouho světla.

it would sound wrong in this context, because we’re not counting several distinct lights; we’re describing the general state of there being light.

Thus, singular světlo is the normal, idiomatic choice here.