In Russian grammar, the adverb theory refers to the study and classification of adverbs. Adverbs are a specific type of word that modify verbs, adjectives, and other adverbs. They provide additional information about time, place, manner, degree, frequency, and cause.

Russian adverbs can be classified based on their meaning and form. Adverbs of time express when an action takes place, such as “сейчас” (now) or “вчера” (yesterday). Adverbs of place indicate where an action occurs, like “здесь” (here) or “там” (there). Adverbs of manner describe how an action is done, for example, “быстро” (quickly) or “неторопливо” (leisurely). Adverbs of degree express the intensity or extent of an action, such as “очень” (very) or “только” (only). Adverbs of frequency indicate how often an action is performed, like “иногда” (sometimes) or “часто” (often). Finally, adverbs of cause explain why an action occurs, for instance, “потому” (therefore) or “из-за” (because of).

By understanding the various categories and forms of adverbs, learners of Russian can enhance their speaking and writing skills, as well as develop a more nuanced understanding of the language.