use的形容词形式是什么

例句：

It has a clean and usable interface and has an Extensions API to extend and customize your installation.

它有一个清爽和可用的'接口，并且还有一个扩展的API可以定制你的安装。

Someone came out with a most useful suggestion.

有人提出了一个非常有益的建议。

Please cancel the words which are useless.

请把无用的词划掉。