It is the difference of the word that you want to emphasize. For example, “彼はジュンです。“ and "彼がジュンです。" Both means are "He is Jun." But, you emphasize object. 　　　　　． "He is Jun." is "彼はジュンです。" On the other hand, you emphasize subject. 　． "He is Jun." is "彼がジュンです。"

there is a difference in nuance. わたしがたべる。（あなたじゃない。） I will eat it .(not you,not anyone) わたしはたべる。（case by case include "あなたはどうする？"） I will eat it .(case by case include "and you?")

This question is asked so often. 「は」 = topic marker 「が」 = subject marker Until you properly understand what is meant by "topic" of a sentence, you won't understand fully the difference here. Be patient, learn a bit more Japanese first, and you'll pick up the difference naturally. You don't need to understand everything tomorrow.