У будинку дев’ятнадцять вікон.

Because у/в meaning in requires the locative case when it shows location.

• Dictionary form: будинок = building / house
• After у: у будинку = in the building / house

So the sentence uses будинку because it means being inside the building, not moving into it.

A useful contrast:

• У будинку = in the building
• У будинок = into the building (movement)

Both у and в can mean in here. Ukrainian often switches between them for euphony—basically, to make the sentence easier to pronounce.

So:

• У будинку = natural
• В будинку = also possible

At the start of this sentence, у будинку sounds smoother because в будинку would begin with a heavier consonant cluster.

Usually, this is not a meaning difference, just a pronunciation-flow difference.

In Ukrainian, the present-tense verb to be is often omitted.

So instead of saying something literally like There are nineteen windows in the building, Ukrainian can simply say:

• У будинку дев’ятнадцять вікон.

Literally, it is closer to:

• In the building, nineteen windows.

This is completely normal Ukrainian.

Yes. That is also correct.

• У будинку дев’ятнадцять вікон. = very natural, concise
• У будинку є дев’ятнадцять вікон. = also natural, a bit more explicit

Adding є can make the existence of something more overt, but in many everyday sentences it is simply left out.

Because after 5 and above, Ukrainian nouns usually go into the genitive plural.

Here:

• вікно = window
• вікна = windows
• вікон = of windows / windows in the genitive plural

Since дев’ятнадцять is in the 5+ group, you get:

• дев’ятнадцять вікон

not дев’ятнадцять вікна.

A very useful basic pattern is:

• 1
  • singular
• 2, 3, 4
  • a special counting form, often looking like the plural
• 5 and above
  • genitive plural

With вікно:

• одне вікно = one window
• два вікна = two windows
• три вікна = three windows
• чотири вікна = four windows
• п’ять вікон = five windows
• дев’ятнадцять вікон = nineteen windows

So дев’ятнадцять follows the same pattern as п’ять, шість, десять, etc.

It is genitive plural.

The noun вікно declines like this in the most relevant forms:

• singular: вікно
• plural: вікна
• genitive plural: вікон

In this sentence, the numeral дев’ятнадцять requires that genitive plural form.

It can mean either, depending on context.

Common possibilities:

• house
• building
• sometimes even something like apartment building if the context suggests it

So У будинку дев’ятнадцять вікон could be translated as:

• There are nineteen windows in the building.
• There are nineteen windows in the house.

If no context is given, building is often the safest neutral translation.

Because Ukrainian has no articles.

So будинку by itself can mean:

• in the building
• in a building
• in the house

The exact English article depends on context, not on a separate Ukrainian word.

That is why one short Ukrainian sentence can correspond to more than one natural English translation.

Yes. Ukrainian word order is flexible, though not random.

This version:

• У будинку дев’ятнадцять вікон.

is a very natural way to present the location first, then the thing being counted.

Other orders are possible, for example:

• Дев’ятнадцять вікон у будинку.

That puts more focus on nineteen windows.

So changing the order usually changes emphasis or information flow, not the core meaning.

A rough English-friendly approximation is:

• de-vyat-NAD-tsyat’

The apostrophe in дев’ятнадцять matters. It shows that the consonant before я is not softened, and that я begins with a y sound:

• в’я sounds like v + ya

So дев’ять is roughly:

• DE-vyat’

and дев’ятнадцять builds on that.

You do not pronounce the apostrophe itself; it just tells you how the sounds are separated.