Breakdown of A janela desta sala é grande.
ser
to be
grande
big / large
a sala
the living room
a janela
the window
Questions & Answers about A janela desta sala é grande.
How is the word desta formed?
It is a contraction of the preposition de (of) and the demonstrative esta (this). Because sala (room) is feminine, we use esta, and combining it with de gives us desta (of this).
Why does desta match sala instead of janela?
Desta (of this) describes the room, not the window! In Portuguese, demonstratives like esta must always match the gender and number of the noun they are pointing directly to, which here is the feminine singular noun sala.
Since janela is feminine, shouldn't grande end in an 'a'?
Unlike many Portuguese adjectives that end in '-o' for masculine and '-a' for feminine, grande (large/big) has only one form. It stays exactly the same whether it describes a masculine or feminine noun.
Why do we use é (from the verb ser) instead of está (from the verb estar) for "is"?
We use ser for permanent characteristics. Being large is a built-in, physical property of this window, not a temporary condition, so we use é.