举例说明：I
will
come
back
in
five
days我五天之内会回来；I
will
come
back
five
days
later我五天后回来；I
will
come
back
after
five
days我五天后回来；I
went
back
five
days
ago.我五天前就回来了；He
went
back
before
than
you.他比你先回来。（先后顺序）。
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