is的過去式是什么？

　　例句：

　　The system was state of the art.

　　這一系統(tǒng)是當時最先進的。

　　It was a time of peak demand for the product.

　　那是對該產品需求最旺的時期。

　　Much of the form was not applicable to me.

　　表格中很多部分不適用于我。