I'm sure you .I'm sure you _________(learn) English well if you write and speak more.

I'm sure you .
I'm sure you _________(learn) English well if you write and speak more.

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.