Last two questions

[Studentlearner]

Is it she and I or her and I? I hear a lot of people say her and I.

I know at the beginning of a sentence it’s the noun and first person. For example, “ May and I are going shopping. My question is, when does it become Mary and me? Does that go at the end of the sentence? For example, the mall was very crowed and Mary and me went decided to go home?

I guess it starting to be two ways of writing and speaking grammar.

[mykwyner]

When pronouns are the subjects of sentences use: I, we, he, she and they. When pronouns are objects of sentences or prepositional phrases use: me, us, him, her and them. (you and it don't change)

The position of the pronoun (beginning or end of a sentence) has no bearing on which to use.

She and I are leaving soon, so we don't have time for that.
We don't have time for that because she and I are leaving soon.
she and I (subjects) are leaving (verb phrase)

For Mary and me, Christmas is a wonderful time.
Santa went for a sleigh ride with Mary and me.
Mary and me are objects of the prepositions for and with.