改错：She sings songs as if she is a bird.

因为要用虚拟语气
as if (= as though) 意思是“好像……,仿佛……”,是个连词,引导方式状语从句.例如：
He makes it seem as if it were one of the best meals he has ever had.
关于as if 的用法比较复杂一些,请注意以下几个问题：
1.as if 引导的从句经常用虚拟语气.从上面的句子可以看出,主句都是现在时态,as if从句都是过去时态,这说明as if 从句表示的意思与事实不符或者相反.例如：
He walks as if he were drunk.
My neighbor treats the child as if he were their own son.
2.但是,如果句子表达的意思与事实并没有什么不符,那么就不使用虚拟语气.例如：
He heard a noise,as if someone was breathing.
It looks as if it is going to rain.
3.as if 经常和look,seem 连用.例如：
She looks as if she is (were) ill,because her face is pale.
It seemed as if there was a fight between the two sides.
4.as if 从句中的主语如果和主句主语相同,而且谓语动词有be,则从句主语和谓语经常省略.例如：
She stood at the gate as if (she was) waiting for someone.
The teacher shouted at the boy as if (he was) angry.
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