为什么过去式有时的谓语动词用be动词,有时用动词过去式

如题所述

当谓语动词是系动词be是,过去式就是be的各种过去式形式(was,were);当谓语动词是行为动词时,就用这些动词的过去式形式,如take的过去式took 。请看例句:
He
was
a
student.(was是系动词)
He
took
the
book
with
him.
(took是take的过去式,表示动作发生在过去)
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