Which word is the correct choice for: My mom and __________ went to the store?

The correct choice is "I" because, in English grammar, the subject of a sentence must be in the nominative case. The phrase "My mom and I" functions as the subject of the sentence, meaning it is performing the action of going to the store.

When combining phrases or subjects, it’s important to maintain the correct grammatical form. "I" is the appropriate form when it is part of the subject. Using "I" conveys that both the speaker (the one saying the sentence) and their mom are the people who went to the store.

In this context, the other options do not fit grammatically as subjects of the sentence. "Me" and "us" are both objective forms used as objects of a verb or preposition, while "we" is a subject pronoun but does not fit the grammatical construction of the phrase when combined with "my mom." Therefore, "I" is the only appropriate choice to maintain correct grammar and coherence in the sentence.