There are numerous explanations for this and it depends on the context of the sentence. Sometimes they are interchangeable and other times, one is more appropriate than the other. The short explanation is that は is a topic marker (but not necessarily the subject of the sentence). が indicate the subject and sometimes the object of the sentence. See the following link for specific examples: http://www.japanese-language.aiyori.org/article1.html

1) 私は田中です。I am Tanaka. 2) 私が田中です。I am Tanaka. 1) The emphasis is on "Tanaka". This is the situation when you are introducing yourself to someone. 2) The emphasis is on the "I". This is the situation where a son believes his father was dead but the person in front of him insists and say " 'I' am the father."

'～が' basically indicates subject, while 'は' not. 「AはBです」does not mean "A is B". It sytactically means "Talking about A, it is B". if you regard ～は as subject, you cannot explain the following sentences. - 「象は鼻が長い」（The elephant has a long nose) which is subject, 象？ 鼻？　 - What's your order? -> 「僕はコーヒー」 　　　（hey, you are a human, not coffee !) - 「あいつは殺す」 　　this means in 99%, "I/We'll kill him" hardly means "He kills" ※ if you say「あいつが殺す」, this 100% means "He kills" - 「男は度胸、女は愛嬌」 　　means: "it's important for men to have courage, for woman, loveliness.", but Men are not courage itself. Men are creatures and courage a state of mind. For more explanation see below. https://www.italki.com/question/424283 https://www.italki.com/question/424839 *** Why '～が' sometimes ’looks like' as 'object? ' It is because Japanese has two layers, English-nized Japanese and Japanese Japanese. https://www.italki.com/question/425077