an adverb is a type of word that modifies or describes verbs, adjectives, and other adverbs. In the case of 'suddenly,' it is used to describe how an action, event, or change happens quickly and without much warning.
rules and use cases for this part of speech involve using the adverb to modify an action verb or certain adjectives.
1. Suddenly, the thunder cracked and rain began to pour from the sky.
2. The intensity of the argument suddenly increased.
3. He suddenly understood the implications of the decision he had made.
'Suddenly' must be used to modify an action verb or certain adjectives (e.g. 'He was suddenly scared' or 'He suddenly looked worried'). Additionally, note that 'sudden' (without the -ly ending) can also function as an adjective to describe something as happening without warning or preparation. Therefore, one might say, 'He experienced a sudden shock' to mean the same thing as 'He was suddenly shocked.'