If a person eats, drinks, or engages in sexual intercourse out of forgetfulness, their fast is not broken. The Prophet Muhammad (peace and blessings be upon him) said:
“If anyone forgets that he is fasting and eats or drinks, he should complete his fast, for it is Allah who has fed him and given him drink.”
(Muslim, “Ṣiyām,” 171)
However, as soon as one remembers that they are fasting, they must immediately stop the act.
If a person, after eating or drinking forgetfully, mistakenly believes that their fast has been broken and continues to eat, they must make up (qaḍāʾ) that day’s fast. But if someone knowingly continues to eat or drink after realizing that their fast is still valid, then both qaḍāʾ (make-up) and kaffārah (expiation) become obligatory.