In what cases is the fast invalidated but only qada is necessary? What actions break the fast but require only qada to compensate? Which circumstances necessitate making up the fast without needing kafara?
Only qada is required for the fasts invalidated due to a valid reason such as travel and sickness. Also, unintentional eating and drinking such as water’s reaching one’s stomach during ablution, and eating the things inedible and against human nature to eat such as raw, uncooked rice and earth which does not appeal human nature as nutrition invalidate fasting and only qada is required (Marghinani al-Hidayah, II, 259, 267-270).
If one starts to observe kaffarah, nadhr and nafl fastings and breaks these, qada is required for them as well.
Unobserved fastings without a valid excuse in Ramadan requires one to perform qada. However, not to fast without any reasonable excuse in Ramadan is a great sin.
Masturbation, unintentionally swallowing raindrop, snow or hail, swallowing water unintentionally while performing ablution, having a bath or swimming, and vomitting mouthful intentionally invalidate fasting and qada is required to be performed (Marghinani, al-Hidayah, II, 256-259, 263).
If a person who is not sure whether it is imsak time yet or not continues to eat and drink and it is understood that the second fajr has risen, then fasting is invalidated and qada is required. Similarly, if one thinks the sun has set and breaks the fast by assuming it is iftar time, but it is understood that it is not iftar yet, then fasting is invalidated and qada is required. (Marghinani, al-Hidayah, II, 277-278)..
If one eats and drinks forgetfully and thinks his fasting is invalidated; or one makes intention in the morning instead of night and thinks the fasting is invalidated and intentionally continues to eat, drink and have sexual intercourse during the day, fasting is invalidated and qada is required.
Source: Presidency Of Religious Affairs The Turkey, High Board of Religious Affairs FATWAS