如果你希望长大后能够使用英语，你可以说"I hope to be fluent in Engpsh when I grow up." 这句话的来源可能是你对自己未来发展的愿望，或者是在某种情境下（比如在学校里）你向别人表达了这个想法。
例句：
"I hope to be fluent in Engpsh when I grow up so that I can communicate with people from all over the world." (我希望长大后能流利地使用英语，这样我就能和世界各地的人沟通了。)
"My parents always tell me to study hard and learn Engpsh well, so that I can have more opportunities in the future." (我的父母总是告诉我要努力学习，学好英语，这样我就能在未来有更多的机会。)
"I want to be able to read and understand Engpsh books and articles when I grow up, so that I can learn more about the world." (我希望长大后能够阅读和理解英语书籍和文章，这样我就能学到更多关于世界的知识。)