答：was只是be动词的过去式之一.
be动词既可作系动词（表示“是”,“在”的意思）后跟表语.表语可以是名词、形容词、介词短语、副词,还可以是非谓语动词形式（动名词doing,现在分词doing,过去分词done,不定式to do)
be 动词还可以作助动词,与现在分词doing一起构成进行时态,与过去分词一起构成被动词态.
下面以was为例说明.
一、系表结构：
He was a worker ten years ago.
He was very happy when he heard that .
He was at home yesterday.
He was out at that time.
以下跟非谓语动词形式作表语：
The film was moving and he was moved.
His job was moving houses for others when he was in his twenties.
Your job that day was to clean the blackboard.
二、助动词
The train was moving slowly.
The tree was moved away.