Questions & Answers about 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.