We use the subject forms when the personal pronoun is the subject
of a finite verb form (a verb form that shows a particular tense, person
and number). Be careful when you have to use a third person singular pronoun:
She is cooking.
He is cooking.
We use the objectforms when the personalpronoun
is:
the object of a verb: John phoned Sheila and asked her
(Sheila) to help him (John) with his homework.
the complement
of a preposition: Jenny was ill and her mother stayed with her
(Jenny) all day.
after
"link verbs". But, in formal and written English, some
people prefer to use subject pronouns after a "link verb: Who is at the
door? It´s me.
Subject 
the object of a verb: John phoned Sheila and asked her
(Sheila) to help him (John) with his homework.