Here's why:
* Pronoun Case: "Us" is the objective case of the pronoun, used when the pronoun is the object of a verb or preposition. "We" is the subjective case, used when the pronoun is the subject of a verb.
* Subject-Verb Agreement: The verb needs to agree with the subject.
Here are some correct ways to use the phrase:
* "Mary and we" (as the subject of a sentence): "Mary and we are going to the movies."
* "Mary and us" (as the object of a verb or preposition): "The teacher gave Mary and us a quiz."
It's important to use the correct pronoun case and ensure subject-verb agreement for grammatically sound sentences.