学员问：be able to，to是介词吗？
Tyger答
第一，be able to和be happy to，be interesting to，be nice to等结构无异。
第二，这里的to根本不是介词，而是分词。
第三，I am able to walk.和I can walk的区别，到底是啥？Tyger编了一个Superboy的小故事来说明问题。
I am SuperBoy. I can walk, I can jump, I can run.
One day, I got a disease. I could't walk, I could't jump, I could't run.
I said to myself：”I want to be a real Super Boy.“
I then made practice every day and I recovered.
I was able to walk.
I was able to jump.
I was able to run.
I become a Super Super Boy now.
第四，进一步思考，为什么会有这个区别呢？关键在于Be able就是一种能力提升过程的表述，can是一种成果成就表述。
