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.那有没有可能加原形的？最后一个例句，is clean看到了吗？ oh, 本应打was clean的。至于不是一个意群的，则不属此类。就是说不定式作表语的话加原形前有do,后无to.我不是举了两个例子吗？这叫作表语用的不定式小品词to的省略。你问问题最好给出例句。我要不是教了这么多年的书，还不能随便举出这么多的例句来。