Why does the past tense of verb (passé composé) sometimes change depending on the pronoun and sometimes not?

The tense known as "passé composé" uses the verbs "être" and "avoir" in their conjugated form and another verb in order to form the past tense of that verb. This tense shows that something has already happened.For example, we can use the auxilliaire avoir:J'ai mangé (I have eaten)Elle a mangé (She has eaten) We can also use auxilliaire être:Je suis alléIls sont allésNotice how auxilliaire être will conjugate the verb depending on the pronouns used. "Ils" suggests many; "sont" comes from auxilliaire être so we must add an 's' to 'allé'. But auxilliaire "avoir" has no effect. 'Elle' is feminine but 'mangé' does not take an 'e' at the end. As a general rule, remember that for auxilliaire 'être', it is important to conjugate the verb depending on the pronoun while auxilliaire 'avoir' will leave the verb as it is.