What's the main difference between saying "Are you sure?" and "Are you serious?" Or are they all interchangeable?

If I use "ever" instead of "could" in this situation, would it change the meaning of a sentence?

No, it would not. "Even" can be used here without changing the meaning of the sentence. When speaking, using "ever" is actually more common than not using it and the sentence flows more naturally as well. Ex: This house has everything you could need. Ex: This house has everything you could ever need.

Why'd he say "have seen the girl" instead of "saw the girl?"

Good question! "Saw the girl," which is past-simple tense, would imply that he saw her a while ago, while they were not in the same room, or at a previous moment. But since they're still in the present moment, the present perfect tense is used. "Have seen the girl" also implies that he has already fully acknowledged and noticed her presence in the space, so the action is already complete. Ex: I've just won the game. => Present perfect tense - which implies that it was very recent Ex: I won the game at the fair. => Past-simple - the time of winning is not clear

Can I use "as the fact that" instead of "for the fact that"?

No, "as the fact that" can not replace "for the fact that".

Is it ok to say "white snow horse" instead of "snow white horse"?

There are two rules to remember when considering the order of adjectives in a sentence. Firstly, they should go in the order of opinion> size > age > shape > color > origin > material > purpose. Ex: A century-old blue vase. => Age > color > material Ex: A ten-year old yellow dog. => Age > color Additionally, when it comes to colors, qualifiers for a color always go before the color itself. Ex: A snow white horse. Ex: It was pitch black outside. Thus, based on these rules, "snow white horse" is correct, and "white snow horse" isn't!

What's the difference between "every day" and "each day"? Are they interchangeable?

"Every day" and "each day" have the same meaning. They both mean that something is occurring daily. You can use these phrases interchangeably. Ex: He goes to the gym almost every day/each day. Ex: Each day/every day, she goes for a run. Ex: We watch t.v. each day/every day.