strange question really.
do you mean when to use i and when to use i am?
I is a subject pronoun, I am is a subject pronoun+a verb to' be' in the present simple tense
i like tea.
i go to the beach in summer.
i always work hard.
i am tired.
i'm thirsty.
i am a teacher.
i'm happy to see you here.
So. I is subject, but I am is a subject and verb