是省略了
补全了应该是:the stronger the value of the dollar【 is】,and the longer it stays strong,the weaker our economic activity 【will be】.
美元的价值越高,持续越久,我们的经济活动就会越低迷.
就像 :the more ,the better
再举个例子:how many books do you want?the more the happier
其实是省略了:the more book I got ,the happier I will be