What is an adverb?

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Adverbs give more information about verbs. They can also be called post- qualifiers, as they give more information after the verb.
Typically, adverbs end in –ly, but this is not a definition of an adverb:
The particles ran quickly. This answers the question: how did the particles run? Quickly, therefore, gives more information about the verb, run. The –ly is a useful marker here but not a rule.
The jet flow was allowed to run fast. This answers the question: how was the jet flow allowed to run? Fast, therefore, gives more information about the verb allowed to run. There is no –ly here but the additional information makes the word function as an adverb.