They go out of date quickly and become tiresome if used too much
请问这里为何是用used？
这里的 if used too much = if used too much
这是条件状语从句中的省略使用方法。类似例句：
In regard to conferences, they are necessary and good, but only good if 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）
