Which vs. That: How to Choose
Is there an easy way to know how to choose?

Question

Answer

If i am understanding your question correctly, and my own understanding is correct, think of it as two seperate sentances:

"Of which is..." the best book. - Of which is singling out the book amungst other books (plural) and saying it is the best.
"That is..." the best book.  - That is the best book is merely saying it is the best book, it could also be the only book!

Answer

I have surrfed many websites ,but I couldnt undersatand ,I mean when to choose one of them in a sentence like this one:

Me: Lately I made some decisions, (which) might change my life

The correction:  Lately I made some decisions, (that) might change my life.

Answer

Why don't you create some examples, or google grammar for which or that?

집에서 편안하게 언어를 배울 수 있는 기회를 놓치지 마세요. 경험 많은 강사진을 살펴보고 지금 바로 첫 수업을 신청하세요!