is的过去式是什么？

　　例句：

　　The system was state of the art.

　　这一系统是当时最先进的。

　　It was a time of peak demand for the product.

　　那是对该产品需求最旺的时期。

　　Much of the form was not applicable to me.

　　表格中很多部分不适用于我。