私は：わたしは：watashi wa you can use this word as a subject of the sentence. ex: わたしは　学生　です。 I am a student. 私の：わたしの：watashi no you can use this word as a word "～ 's"( possessive case）. ex: それは　わたしの　ボールです。Its my ball. 私に：わたしに：watashi ni you can use this word as an object of the sentence. ex: あなたは　私に　本を　くれた。 You gave me a book. / You gave a book to me.