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?

Non perdere l'opportunità di imparare una lingua comodamente da casa tua. Esplora la nostra selezione di insegnanti di lingue esperti e iscriviti subito alla tua prima lezione!