I have been separated (not divorced) from my wife since August 1st. She has already filed her taxes and has informed me that she has filed "head of household." Doesn't that mean she has filed as "single?"

I thought that we were required to file "Married Filing Separately or Joint?"

Shouldn't I be able to also claim one of the children?

I have not filed yet because I do not want to do something wrong. She is expecting me to just file single. What happens if she files one way and I file another?