Spanish relative adverb
A relative adverb introduces a group of words, or a clause, that tells more about a noun. Relative adverbs can be used instead of a relative pronoun plus a preposition. There are three main relative adverbs: where (donde), when (cuando), and why (por qué).
The relative adverb cuando means "when", “in which” or “at which” and is used to refer to a time expression.
Ahora es cuando le pides disculpas a ella.
Now is the time when you apologize to her.
The relative adverb por qué means "why" or “for which” and is used to refer to a reason.
No sé por qué Pedro no está en clase hoy.
I don't know the reason why/for which Peter isn't in class today.
The relative adverb donde means "where", "in which" or "at which".
Estoy en un bosque inmenso, donde hay muchas plantas tropicales.
I am in an immense forest where there are many tropical plants.