How can one be certain that the liquid reaching the mouth from the throat is stomach acid, which necessitates breaking the fast, and is everything that reaches the mouth from the throat considered to break the fast?

If something from the stomach reaches the mouth—whether vomit, regurgitation, food, or phlegm—in such a way that it could be expelled, and the fasting person intentionally swallows it, their fast is broken. However, if it does not reach the mouth but returns to the stomach, the fast is not broken by that. What is felt in the throat, if it is merely an illusion, should be disregarded. But if you are certain that a liquid from the throat reached the mouth in such a way that it could be expelled, then intentionally swallowing it breaks the fast. If done out of forgetfulness or ignorance, the fast remains valid.