【高中英语】can be able to和will be able to区别?

【高中英语】can be able to和will be able to区别?
一道完成句子题：
Only when you have read a lot ____ a good article.(able)
只有当你进行了大量的阅读时你才能写出好文章.
答案是will you be able to,而且特别强调了不要用can you be able to,说can u be able to是很不规范的用法.
求问为什么QAQ?
谢谢=v=!