En ese edificio hay veinticuatro ventanas, y por la noche se ve muy bonito desde fuera.

Ese means that. In modern Spanish, the basic three-way contrast is:

• este = this
• ese = that
• aquel = that over there / that one further away

So ese edificio means that building. Also, demonstratives like ese are normally written without an accent mark in current standard Spanish.

Hay is used to say that something exists or is there somewhere: En ese edificio hay veinticuatro ventanas = There are twenty-four windows in that building.

You could also say Ese edificio tiene veinticuatro ventanas, which means That building has twenty-four windows. Both are correct, but hay is more about existence/location, while tiene is more about the building as the possessor of that feature.

Because hay is an impersonal form of haber. It always stays the same, whether what follows is singular or plural:

• Hay una ventana
• Hay veinticuatro ventanas

So even though ventanas is plural, hay does not become plural.

In Spanish, the numbers from 21 to 29 are usually written as one word:

• veintiuno
• veintidós
• veintitrés
• veinticuatro

So veinticuatro is the normal spelling. Some of these numbers have accent marks, but veinticuatro does not.

Por la noche means at night or during the night. In Spanish, especially in Spain, por is very commonly used with parts of the day:

• por la mañana = in the morning
• por la tarde = in the afternoon/evening
• por la noche = at night

En la noche exists, but in Spain it is less natural in a sentence like this.

Here se ve means it looks or it appears. So se ve muy bonito is a very natural way to say it looks very pretty.

This use of verse is common when talking about how something looks from a certain point of view:

• Se ve bien = It looks good
• Se ve mal = It looks bad
• Se ve bonito = It looks pretty

So this is not really a literal reflexive see oneself idea in English; it is just how Spanish often expresses appearance.

Because bonito agrees with the thing that is understood as looking nice: the building (el edificio), which is masculine singular.

It does not agree with ventanas here. If the sentence meant the windows look very pretty, then you would expect:

...se ven muy bonitas

But in your sentence, the natural idea is that the building looks nice from outside at night.

Because muy is used before adjectives and adverbs, while mucho is usually used with nouns or as an adverb meaning a lot.

So:

• muy bonito = very pretty
• muchas ventanas = many windows
• llueve mucho = it rains a lot

Since bonito is an adjective, muy is the correct choice.

Desde fuera means from outside or from the outside. It tells you the viewpoint from which the building looks nice.

• fuera = outside
• desde fuera = from outside

So se ve muy bonito desde fuera means that it looks nice when you see it from the exterior. The phrase desde makes that point of view explicit.

Not really. In Spanish, a comma before y is often omitted, so many people would write:

En ese edificio hay veinticuatro ventanas y por la noche se ve muy bonito desde fuera.

The comma is not impossible here, because the second part is a separate idea and there is a natural pause, but without the comma is more typical. So as a learner, it is safest not to put a comma before y unless you have a specific reason.