Interrogative Adverbs in Icelandic grammar are words used to ask questions about manner, time, place, reason, and quantity. They play a significant role in adding meaning and providing depth to a sentence. Icelandic Interrogative Adverbs include words such as “hvernig” (how), “hvenær” (when), “hvar” (where), “afhverju” (why), and “hversu” (how many). These words add a certain level of inquiry to the sentences, helping learners to formulate questions in the Icelandic language.