stop......from(doing) sth=prevent......from(doing) sth=keep......from(doing) sth

因为 stop sb. (from) doing sth 和 prevent sb (from) doing sth 省掉了from 不会影响语义，而 keep sb. from doing sth 省掉from后，意思正好相反：
I kept him working on his homework —— 我一直让他做作业
I kept him from working on his homework —— 我不让他做作业