1,I like writing my own original sentences______the grammar I've learned.
选择 B to use
to use the grammar I've learned 是不定式用作目的状语,意思是“为了/以便使用我学过的语法”
2,I started to write my own original sentences______the grammar I was learning.
选择 D using
using the grammar I was learning 是分词短语作方式状语,意思是“用我正学着的语法(这种方式)”