Someone had (ate, eaten)all the brownies.

=eaten

Right.

Thanks:)

You're welcome.

The correct word to use in this sentence is "eaten."

To determine the correct form of the verb "eat" to use in past tense, you need to consider whether the action is completed or ongoing. In this case, the action of eating all the brownies is completed, which means you should use the past participle form of the verb, which is "eaten."

To form the past participle of regular verbs like "eat," you typically add "ed" to the base form of the verb. However, "eat" is an irregular verb, and its past participle form is "eaten." So, in this case, it would be grammatically correct to say, "Someone had eaten all the brownies."