i got a phone only then did i know
after they came in, only a doctor they saw
问题：为什么上下2个句子一个有did，一个没有？是不是因为倒装的东西不同引起的
再问一个问题：
what do u like?
---Pingpong. What fun it is to play in the open air.
可以说：how fun it is...吗？
第一句是倒装。但第二句有一点：当only强调人，物，代词，名词时不倒装．而且不能用在否定句子里。 第二句强调 a doctor, 不倒装。这一句实际上是after they came in, there is only a doctor (that/whom) they saw.这是个人理解。供参考。关于补充问题： what do u like?---Pingpong. What fun it is to play in the open air.【我觉得这里已经错了。what + 名词，正确的应该是What a fun it is to play in the open air.】然后可以换成：how fu
