is - an的过去式

is的过去式是was，an是不定冠词，没有过去式，例如：

1、This model was two years in the making.这种型号是用两年时间制成的。

2、The new law was introduced in 1991.这项新法律是于1991年开始实施的。

3、It was concluded that the level of change necessary would be low.结论是，需要作出的变更程度很低。