Parents complained that some of the substitute teachers were clearly unqualified.

unqualified 是由前缀 un-（表示"不、非"）和形容词 qualified（合格的）组成的形容词。qualified 来自动词 qualify（使具有资格），所以 unqualified 的字面意思是"不具备资格的"，即"不合格的"。