suppose 在什么情况下要用虚拟语气？如Nancy is supposed to (做完化学实验)　at least two weeks ago.

Nancy
is
supposed
to
(做完化学实验)　at
least
two
weeks
ago.
这句话不是用不用虚拟语气，因为是不定时用现在时还是完成时的问题,单独来看2种情况都可能成立，适用于不同语境下。
at
least
two
weeks
ago至少两周以前，这句话有责备的含义，使用完成时表示她应该做什么（而没做），2周前她就被告知做什么。Nancy
is
supposed
to
have
finished
the
chemistry
experiments
at
least
two
weeks
ago.
Nancy
is
supposed
to
finish
the
chemistry
experiments
at
least
two
weeks
ago.
责备的含义就弱了很多。差别在这里。
希望对你有所帮助。