Yahoo奇摩字典文法標註表稱,標註為 [F] ︰該形容詞只用作或主要用作述語(predicative) ，置於連繫動詞之後。 例：(正) John is .　(誤) John is a drunk man. 何解?謝謝?

Your case refer to modal verbs in Chen's The heart of Grammar.
The adjective is used only or primarily as a predicative after the modal verb in the predicate. Here Modal verbs are used with other verbs to change their meaning in the sentence in various ways; ie:-ought to---used to---don't need ---would like----can--may--shall---will---must---need&dare etc.
eg:-"John is" is similar to "You ought to"---"They used to"-----"do need---"would like".
eg:-You ought to go.
You ought to work harder.
They used to live in Tainan.
They used to live there.
They don't need to go yet.
They would like to employ some servants.