what do you think is the most difficult……这个用法对吗？

what
do
you
think
is
the
(difficult
)
{difficult}
subject?
因为在subject的前面，所以是形容词性，填原形

这个用法是对的。
What
do
you
think
is
the
most
difficult？
do
you
think是，what是的主语时就可以这样用的。
What
is
the
most
difficult?什么是最难的？
What
do
you
think
is
the
most
difficult？你认为什么是最难的？
疑问词不是疑问句的主语时，要注意疑问句就再也不能用疑问句语序了，因为do
you
think已占用。
如：
What
do
you
think
they
are
doing?
疑问句：
What
are
they
doing?