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?

Não perca a oportunidade de aprender uma língua no conforto da sua casa. Navegue pela nossa seleção de professores de línguas experientes e inscreva-se na sua primeira aula agora mesmo!