I'm sure you ................

题目的意思是:我肯定你会把英语学得很好如果你多写多说的话.
从题意可知道这件事情还没有发生过...所以因该用将来式
所以是:will learn
I'm sure you {will learn} English well if you write and speak more.

I'm sure you learn(learn) English well if you write and speak more.
我确信你能英语学得好如果你能更多的写和说。
这句是说话人的一个普通陈述，表示说话人的主观意愿。用动词原形。

I'm sure you will learn English well if you write and speak more.

will learn

would learn