He paid the boy for washing ten windows,most of them hadn't

He paid the boy for washing ten windows,most of them hadn't been cieaned for at least a year.
上面的是两句话,中间缺少连接词.
可以这样改：
He paid the boy for washing ten windows,most of which hadn't been cieaned for at least a year.
这样改就是定语从句了,而你原句是两句话,在英语中,两句独立的话之间必须有连接词.
再问： 他写了20部小说，这部是最成功的。 那么这个用定语从句翻译你会吗
再答： He wrote 20 novels, one of which is most successful.