Relative Adverbs in Vietnamese grammar, known as “Trạng từ quan hệ” are used in complex sentences to refer back to a place, time, or reason in the principal clause. They’re commonly used when constructing cohesive, complex ideas and often replace a preposition and a pronoun combination in English. The several relative adverbs in Vietnamese include “đâu” (where), “khi nào” (when), “vì sao” (why), among others.