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

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