Мальчик рисует в альбоме у окна.

Russian does not have articles. So мальчик can mean the boy or a boy, depending on context.

The same is true for the other nouns:

• в альбоме = in the album / in an album / in the sketchbook
• у окна = by the window / by a window

Russian usually leaves definiteness to context rather than marking it with a separate word.

Мальчик is in the nominative singular because it is the subject of the sentence — the person doing the action.

Here:

• мальчик = the boy
• рисует = is drawing / draws

So мальчик is the basic dictionary form of the noun.

Рисует is the 3rd person singular present-tense form of рисовать.

So it means:

• he draws
• he is drawing

Russian present tense does not normally distinguish between draws and is drawing the way English does. The context tells you which is more natural.

Also, рисовать is imperfective, so it presents the action as ongoing, repeated, or simply in progress, not as a completed result.

Because в can take different cases depending on the meaning.

Here it means in as a location, so Russian uses the prepositional case:

• альбом → в альбоме

So в альбоме means in the album / in the sketchbook.

Compare:

• в альбоме = in the album, inside it, on its pages
• в альбом = into the album

A good rule:

• where? → often prepositional after в
• where to? → often accusative after в

Not necessarily in the modern English sense of a music album.

In Russian, альбом can mean:

• an album for photos
• a sketchbook or drawing pad
• a book with blank pages for collecting or drawing things

So in this sentence, в альбоме most naturally means something like in a sketchbook or in a drawing book.

У means by, near, next to here, and у requires the genitive case.

So:

• окно = window
• у окна = by the window

Окна is the genitive singular form of окно.

This is very common:

• у дома = by the house
• у стола = by the table
• у окна = by the window

It usually means by the window or near the window.

It suggests location next to the window, not literally touching it.

Some useful contrasts:

• у окна = by the window
• на окне = on the window / on the windowsill
• в окне = in the window

So у окна is the natural choice for someone sitting or standing near a window.

Yes. Russian word order is much more flexible than English word order because the case endings show the grammatical roles.

The neutral order here is:

• Мальчик рисует в альбоме у окна.

But you could also say:

• У окна мальчик рисует в альбоме.
• В альбоме мальчик рисует у окна.

These versions still mean roughly the same thing, but the emphasis changes:

• putting у окна first highlights the location
• putting в альбоме first highlights where he is drawing

So the original sentence sounds neutral and natural.

Most naturally, it describes the whole situation, especially the boy’s location while drawing.

So the normal interpretation is:

• the boy is drawing in his sketchbook
• and he is by the window

In theory, language can sometimes be structurally flexible, but in ordinary use a listener will almost certainly understand у окна as the place where the drawing is happening, not as a special description of the album itself.

The standard stress is:

Ма́льчик рису́ет в альбо́ме у окна́.

A rough pronunciation guide:

• Ма́льчик = MAL-chik
• рису́ет = ree-SOO-yet
• в альбо́ме = v al-BO-me
• у окна́ = oo ak-NA

Knowing the stress matters, because Russian stress is not always predictable.