表语形容词通常用不定式作补足语。但comfortable作表语如果用不定式作补足语，则不定式必须为悬空不定式，即句子主语为不定式的逻辑宾语。例如：The clothes are comforable to wear. 所以如果comfortable作表语需要补足，而句子主语与非谓语动词为主动关系时，就不能用不定式作形容词补足语，而用in doing sth或doing sth. 如果不用介词，则doing sth分析为现在分词作comfortable的补足语。句型为：
Sb is comfortable (in) doing sth.
= sb feels confident in doing sth.
