"I'm leaving tomorrow" this form is a future arrangement( or Present continues ) : is used to express a future arrangement between people . It usually refers to the near future .
" I'm going to leave tomorrow. " Going to is used to express a future plan , decision , or intention made before the moment of speaking .
2.(1) When I was leaving room, the phone rang.
(2) When I was just going to leave room, the phone rang.
(1) means that you were actually walking out of the room. (2) means that you were intending to leave almost immediately, but you hadn't yet started to walk out.
In other words, the first means you were in the process of leaving the room, the second simply implies intent to leave the room, but could also have the implication of one being in the process of leaving the room.
4.的確，許多台灣的文法書寫到「be going to」後面應該避免再加上go、come、leave等字。 但本公司外籍教師認為在現今美語此規則並未存在。 例如，我們在許多國外教科書都發現有「be going to go」的用法：
▲ I'm going to go to the beach. (Jack Richards主編 New Interchange 2 p. 29 Cambridge 出版社2005出版) ▲ Are you going to go to the movies? (Michael McCarthy主編Touchstone 2 p. 37 Cambridge 出版社2005出版) ▲ We're going to go somewhere for dinner. (Michael McCarthy主編Touchstone 2 p. 37 Cambridge 出版社2005出版) ▲ I just finished my homework. Now I'm going to go shopping. (大家說英語2002年8月號 p. 30)
本公司外籍老師認為：若有情境的話,「going to go home」和「going home」是有差別的。如你看到你同事在整理包包,你猜他準備要回家了,你可以詢問他：Are you going to go home?或者Are you going home?。但你若是路上，但到你同事行色匆匆，你大概只會問他Are you going home?。