Снаружи уже были сумерки, а изнутри книжного магазина лился тёплый свет.

Russian does not use a dummy subject like English it in sentences such as It was dark, It was late, or It was twilight.

So English It was already twilight outside becomes simply:

Снаружи уже были сумерки.

The idea is complete without any pronoun. Russian often leaves out a subject when English would use it only for grammar.

Сумерки is normally a plural-only noun in Russian. It refers to twilight / dusk, but grammatically it behaves like a plural noun.

So you say:

• были сумерки = it was twilight
• not было сумерки

This is similar to how some English nouns are formally plural, even when they refer to one idea, like scissors or pants.

Because сумерки is plural, the past-tense verb must also be plural:

• сумерки были

Уже means already.

In this sentence, it suggests that dusk had already set in by that point. It adds a sense of time progression:

• Снаружи были сумерки = Outside, it was dusk.
• Снаружи уже были сумерки = Outside, it was already dusk.

So уже helps create atmosphere: outside it had already grown dim, while inside there was warm light.

Снаружи means outside / on the outside. It focuses on the contrast between outside and inside.

That makes it especially natural here, because the second clause has изнутри (from inside). The sentence is built around that opposition:

• снаружи = outside
• изнутри = from inside

By contrast, на улице means out on the street / outdoors, and it sounds a bit more literal and location-based.

So:

• Снаружи уже были сумерки = Outside, it was already twilight.
• На улице уже были сумерки would also be possible, but it shifts the feel slightly toward outdoors/in the street rather than a neat inside-outside contrast.

А often marks a contrast or juxtaposition: one thing is true, and another contrasting thing is also true.

Here the contrast is very natural:

• outside: twilight
• inside: warm light

So а works very well:

Снаружи уже были сумерки, а изнутри книжного магазина лился тёплый свет.

Why not the others?

• и = and; it would just connect the two facts, with less contrast
• но = but; it would sound like a stronger contradiction than the sentence really needs

So а is the best choice for a calm visual contrast.

Изнутри means from inside or from within.

In this sentence, it introduces the source of the light:

изнутри книжного магазина = from inside the bookshop

It helps create a visual image: the warm light is not just coming from the shop, but specifically from its interior.

Because изнутри here functions like a word meaning from inside of, it is followed by the genitive.

So:

• книжный магазин = a bookshop / bookstore
• книжного магазина = of the bookshop / from inside the bookshop

The adjective also changes to match:

• книжный → книжного
• магазин → магазина

This is just normal case agreement in Russian.

Yes, you could, but the nuance is a little different.

• из книжного магазина = from the bookshop
• изнутри книжного магазина = from inside the bookshop / from within the bookshop

Изнутри is more vivid and spatial. It emphasizes the contrast with снаружи and makes the image more cinematic: darkness outside, warm light pouring from within.

So изнутри is stylistically stronger here.

The verb литься literally means to pour, to flow, or to stream. Russian often uses it figuratively with things like light, music, or sound.

So:

лился тёплый свет
literally: warm light was pouring/streaming

This is more expressive than simply saying:

• был тёплый свет = there was warm light

It gives a soft, flowing image of light coming outward from the shop.

Also, лился is masculine singular because свет is masculine singular:

• свет → лился

Because свет is the subject of the second clause, so it stands in the nominative case:

• тёплый свет = warm light

The verb agrees with it:

• лился тёплый свет

By contrast, книжного магазина is genitive because it depends on изнутри.

So in the same sentence you see two different case patterns:

• книжного магазина = genitive
• тёплый свет = nominative

Yes, but in a natural, stylistic way.

The sentence begins with the setting and contrast:

• Снаружи уже были сумерки
• а изнутри книжного магазина лился тёплый свет

This order helps paint the scene before naming the light itself. Russian word order is flexible, and this version sounds smooth and descriptive.

For example, you could also say:

Тёплый свет лился изнутри книжного магазина.

That is grammatically fine, but it is a bit less atmospheric. The original version foregrounds the contrast between outside and inside, which is one of the main effects of the sentence.