know all the answers 自恃聪明；聪明伶俐；自命为万事通

【例句】
Our headmaster seems to know all the answers.
我们的校长看起来知识渊博。

John seems to know all the answers, but I don't think he's as clever as he appears at first.
约翰似乎是个万事通，不过我认为他没有看上去的那样聪明。