fly off on a tangent

• 突然离题；突然离开话题

例句

(ph.)

1. We were discussing last night's concert when Anna went off at a tangent and started talking about her new flat.
   当我们正讨论昨晚的音乐会时，安娜突然离开话题，开始谈起她的新公寓来了。
2. It is difficult to have a sensible discussion with my mother because she's always flying off at a tangent and talking about something else.
   同我母亲进行有成效的讨论很困难，因为她总是突然离开话题，扯到其它一些事情上去。