条件状语从句 if 的用法 — 英语填空题
题目
用括号内所给动词的适当形式填空：The old man always tells us that we won't find the answer to the problem unless we think about it carefully. In fact, if we ___ (keep) trying in a different way, we will surely find a wise solution sooner or later. If we give up halfway, we ___ (waste) all the time we have spent.