Los adverbios relativos introducen oraciones relativas que precisan ideas o lugares o que precisan el tiempo. Los adverbios relativos en inglés son: what, where, when, why. En general se pueden sustituir estos adverbios relativos con una preposición + which.

Adverbios relativos
This is exactly what I told you this morning.
Esto es lo que yo te dije hoy por la mañana.
The church where our parents got married is somewhere in the mountains.
The church, in which our parents got married, is somewhere in the mountains.
La iglesia donde mis padres se casaron está en alguna parte de la montaña.
The time when he leaves for vacation, we will be happy to be rid of him.
The time in which he leaves for vacation, we will be happy to be rid of him.
Cuando esté de vacaciones, seremos felices de habernos liberado de él.
The reason why she couldn't come to your party is that she was really, really sick.
The reason, for which she couldn't come to your party is that she was really sick.
La razón por la cuál no podía venir a vuestra fiesta es que estaba enferma.