I wish I were here yesterday中为什么不用was

I wish I were here yesterday中为什么不用was I wish he were here.为什么是were不是was阿 Were you here yesterday?Yes,i（ ） am、was、were? I wish you were here I ____ you a good grade I _____ I were here yesterday.(wish 下面这句话,为什么是I wish I was 而不是 I wish I were? 为什么是I wish I were a bird 而不是I wish I was a bird 为什么If only I were abird.中的were不用was 为什么 if i were a boy 中的were 不用was? If I were a boy中,为什么要用were不用was? 歌词sometimes i wish i were an angle为什么这里用的是were不是was. I WISH I WERE A BIRD.怎么这句子里面用WERE,不用WAS啊?是不是书上写错了