The construction of the new pool was to have been completed last month,but the long spell of rainy w

题目

The construction of the new pool was to have been completed last month,but the long spell of rainy weather has postponed the opening until October.
请问was to的作用是什么 为什么不能直接写作has been completed last month?

答案

Was to 在这里是应当的意思,也就是说该工程理应完工,却因为下雨延误了.如果直接用have been,就等于说工程的确是按时完工了,在意思上来说和后面那句话也说不通了.