Millie wasn't in the library yesterday afternoon.
Was Millie in the library yesterday afternoon?
Yes, she was.
句子含有be动词was,故否定句需在was前+not,常用缩写:wasn't. 其余结构保持不变。故答案为:Millie wasn't in the library yesterday afternoon.
一般疑问句需将was置于句首,其余结构保持不变。故答案为:Was Millie in the library yesterday afternoon?
一般疑问句的回答,需用与问句引导词保持一致,故用was,故答案为:Yes, she was.