单复数 时态 英语they saw a number of sheep were eating crops in the

单复数 时态 英语
they saw a number of sheep were eating crops in the field by the hill.the number ()twelve at least.请问为何填was 而不是were 还有麻烦请翻译最后一句话
When and where to build the new factory () yet.答案为什么是 is not decided 而不是 has not decided 此句的含义不是 何时何地建这个新工厂还没有决定 我认为根据这句话的含义 应该用现在完成时 况且这里还有yet