In her book, Marie Kondo show us a beautiful framework to help usfor determineing whether to keep something or to not.

My correction sounds very natural and sophisticated, I have seen this word order whilst reading !

This framework is so simple butyet so powerful: asking yourself if thatkeeping it would makes you happy.

You do not need the ‘so’ before powerful, it just sounds a bit more artful !

You see, how many times have we keept something that doesn't makes us happy just for the fear of losing it?

Tenses !

This is a beautiful Japanese perspective of tifdying up: not focusing aboutit invites us to focus not on how useful, or expensive or small are the objects but if theysomething is, but rather on if it brings us joy.

Very sophisticated and concise stylistic correction, that expresses exactly what you wanted to say as a native would !

She also includes some tidying tips like starting with the less sentimental objects and then with those with more sentimental onesvalue, but the principal idea of the book is thatis.

In her book, Marie Kondo shows us a beautiful framework to help us determine whether to keep something or not.

This framework is so simple but powerful: asking yourself if thatwhether something makes you happy.

You see, how many times do we keep something that doesn't makes us happy just for the fear of losing it?

This is a beautiful Japanese perspective of tifdying up: not focusing abouton how useful, expensive or small are the objects but ifwhether they bring us joy.

She also includes some tidying tips like starting with the less sentimental objects and then withmoving on to the sentimental ones, but the principale idea of the book is thatis.

In her book, Marie Kondo shows us a beautiful framework to help us determine whether to keep something or not.

This framework is so simple but, yet so powerful: asking yourself if thatsomething makes you happy.

You see, how many times do we keep something that doesn't makes us happy, just for the fear of losing it?

This is a beautiful Japanese perspective of tifdying up: not focusing abouton how useful, expensive or small are the objects but ifare, but whether they bring us joy.

She also includes some tidying tips like starting with the less sentimental objects and then with theobjects of less sentimental onesvalue, but the principale idea of the book is thatremains.

At least that's what I think you meant. Btw: principal = head-master of a school.