In Catalan grammar, adverbs of frequency are used to indicate how often an action or event occurs. These adverbs can be placed before or after the main verb, depending on the context and emphasis. Common adverbs of frequency in Catalan include "sempre" (always), "sovint" (often), "normalment" (usually), "rarament" (rarely), and "mai" (never). These words are essential when learning Catalan, as they help to convey the frequency of actions and create more accurate and detailed sentences.