Question

"You must have watched that football match last night,didn't you?"后面的疑问句为什么不是hadn't you?

Answer 1

陈述部分有must
的疑问句，疑问部分根据实际情况而定。(我们便要分析一下must的含义。如果must
作“一定；要；必须”讲，反意疑问句须用mustn't或needn't；而当must作推测意义“一定是；必定”讲时，反意疑问句则需根据must后的动词原形选用相应的形式)
He
must
be
a
doctor,
isn't
he?
You
must
have
studied
English
for
three
years,
haven't
you?
/
didn't
you?
He
must
have
finished
it
yesterday,
didn't
he?

have(has)不是表示“有”的意思，并在句中做谓语时，其反意疑问句的助动词要用do,
does,
did。