it is easy to take care of为什么后面没有it

比较以下句子,再研究一下,你的问题就解决了：
This book is interesting to read == It is interesting to read this book
It is easy to do this job== This job is easy to do.
You are easy to teach=== It is easy to teach you
These old people are difficult to take care of === it is difficult to take care of these old people.
--------现在明白了吗?a good day to you.------------