答：它们是be动词的过去式
一、作助动词：
1. 现在分词,构成过去进行时态：
He was reading.
2.过去分词,构成被动语态：
They were defeated.
二、作系动词
3. 现在分词、动名词短语作表语：
The film was moving.
His job was teaching English .
4. 过去分词作表语：
We were moved.
5.不定式作表语：
1）有to不定式：His work that day was to clean the blackboard.
2) 无to 不定式：What he should do is clean the blackboard.