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。
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。
