Why cant you use "us" in the beginning of a sentence?
English is a subject-first language. That is, it has SVO word order: Subject + Verb + Object. The pronoun "us" functions as an object. It comes after the verb, never before the verb.
Subject Pronoun: We ate the cake. ("ate" is the main verb)
Object Pronoun: You made us eat the cake. ("made" is the main verb)