1. She doesn't eat breakfast every day.

[She eats breakfast sometimes, but she skips breakfast sometimes. In this case, we can use this expressions, can't we?]

2. She doesn't eat breakfast at all every day.
[If we put 'at all', she doesn't eat breakfast any day. Right?]

1. Yes, you are right.

2. Yes, right again -- and because she doesn't eat breakfast at all, you don't need to add "every day." The "at all" takes care of that!

Okay, thank you.

1. In the sentence "She doesn't eat breakfast every day," the use of "every day" indicates that she does not eat breakfast on a daily basis. It implies that she may eat breakfast on some days, but skips it on other days. So, yes, you can use this expression to convey that she eats breakfast sometimes but skips it sometimes.

2. In the sentence "She doesn't eat breakfast at all every day," the addition of "at all" intensifies the negation. It means that she does not eat breakfast on any day, emphasizing that she completely avoids breakfast every day. So, you are correct in understanding that by adding "at all," it conveys the meaning that she does not eat breakfast any day.