Answer: The word that is the understood subject in the sentence "Run!" is "you".
Explanation: The subject of a sentence is the noun, pronoun or noun phrase that indicates what or who is going through a process, performing an action or experiencing a feeling. One type of subject is the understood subject, which is not explicitly written in a sentence, but it is clearly understood since "you" is always the subject of an order or a suggestion. In that way, understood subjects take part in a sentence when the imperative mood is used. The sentence "Run!" is in the imperative mood and it expresses a command. Taking everything into account, although the subject has not been explicitly written, it is clearly understood that the subject is "you".
