老师能帮忙解答一下as if的用法吗？什么时候用假设，什么时候不用呢？我好疑惑

Everybody ignored me as if I didnt exist. 时态一致，没用假设？为什么不能用hadnt existed?He ate as if he hadnt eaten for a week.时态不一致，用了假设He acted as if I was dead. 这个句子是对的吗，为什么不能用hadnt been dead?He is acting as if everything is normal. 时态一致，为什么不用假设