What grammatical error exists in the sentence about Brian and the bus?

The identification of "run quick" instead of "run quickly" as the error highlights the importance of using the correct adverb form in English grammar. In this context, "quickly" serves as the appropriate adverb to modify the verb "run." Adverbs enhance verbs, adjectives, or other adverbs by providing more detail about how an action is performed. Since "quick" functions as an adjective, it cannot correctly modify the verb "run." Therefore, the proper phrase should be "run quickly" to communicate that the action of running is being done in a quick manner.

The other options do not accurately address the grammatical error present in the sentence about Brian and the bus. The phrasing related to “shouted to his brother” might be grammatically acceptable depending on context, while "will leave likely" is less typical in formal English—though it could arguably suggest an informal speech pattern rather than a strict grammatical error. Hence, focusing on the need for correct adverb usage clarifies why the choice about "run quick" is considered the key error.