Using a non-finite verb form: A lot is known about Van Gogh's life and his feelings because of the hundreds of letters， which were written by him to his brother Theo and others.

Using a non-finite verb form: A lot is known about Van Gogh's life and his feelings because of the hundreds of letters， which were written by him to his brother Theo and others.

A lot is known about Van Gogh's life and his feelings because of the hundreds of letters written by him to his brother Theo and others.