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.
【正确答案】：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.
【题目解析】：non-finite verb 是非限定动词，在句子中不能单独用作谓语，不受主语的人称和数的限制，在句子中可以用作其他句子成分。非限定动词有三种：不定式、动名词和分词。例如：Scientists hope to find a cure for cancer.其中的to find 就是非限定动词。