虚拟语气如何准确判断是与过去相还是和将来现在相反呢？

英语中的所谓“虚拟语气”有两种含义：1）与事实相反，这一种含义多见于对已经过去的事件的叙述；2）表达意愿或者可能性，这一种则多用于尚未发生的事情或行为中。
wish
就是一个很有代表性的动词。有些词书把wish和虚拟语气联系起来，主要是因为这个词一般都与虚拟语气的表达结构一起用。然而其实际语义要结合上下文和句子的时态来判断属于哪一种意义。
还有一点需要说明的是，在讲英语的环境里，虚拟语气的句法结构还常见于礼貌请求、客气表达中，如：
i
was
wondering
if
you
could
drop
me
a
few
lines...（请求。事情尚未发生）
i
wish
i
could
participate
in
this
event.（意愿。事情尚未发生，仍有发生的可能）
真正意义上的“虚拟”：i
wish
i
could
have
had
that
opportunity.（事情已过，不可能再发生了）

虚拟语气可以表示过去，现在和将来的情况，时态的基本特点是时态往后推移。
a.
与现在事实相反的假设
则用一般过去时（be用were）should（would）等
+动词原形
例如：If
they
were
here,
they
would
help
you.
如果他们在这儿，会帮助你的。
含义：They
are
not
here,
they
can’t
help
you.
b.
与过去事实相反的假设
则用过去完成时
should（would）等+
have+
过去分词
例如：If
he
had
come
yesterday,
I
should
/
would
have
told
him
about
it.如果他昨天来的话，我会把这件事告诉他的。
含义：He
did
not
come
yesterday,
so
I
did
not
tell
him
about
it.
c.
表示对将来不大可能发生的事情的假想
则用一般过去时
should/would等
+
动词原形
were+
不定式
should+
动词原形
例如：If
you
succeeded,
everything
would
be
all
right.
如果你将来成功了，一切都会好的。
If
you
should
succeed,
everything
would
be
all
right.
If
you
were
to
succeed,
everything
would
be
all
right.
含义：You
are
not
likely
to
succeed,
everything
will
be
what
it
is
now.