They go out of date quickly and become tiresome if used too much
请问这里为什么是用used?
这里的 if used too much = if (they are) used too much
这属于条件状语从句中的省略用法。类似例句:
In regard to conferences, they are necessary and good, but only good if (they are) followed by action. 就会议而言,开会是必要而又有益的,但只有在会后付诸行动的会议才是有益的。(赵俊英:《现代英语语法》p.1507)