Orders and issues pertaining to Qaza (repetition) of missed Roza's (Fasts)

1) If a person forgetfully eats or drinks something during the Roza then in such a case there's no Qaza or expiation on it, whether it's a Roza of Ramadan or not. 

2) If a person in a state of Roza, willingly eats or drinks, then on it; Qaza and Kaffara (expiation) is liable. 

3) And if a fasting person in the holy month of Ramadan due to some reasonable circumstances such as in journey or because of sickness, breaks the fast, then only Qaza will be liable on him/her but not the Kaffara. 

The order of Kaffara is only related to the Roza's of Ramadan and if someone beaks the Nafl Roza (Optional Fast) due to some reason then in such case there's no expiation liable on Him/her. Only performing the Qaza is sufficient. 

Allah (Subhanahu Wa Ta'ala) said: No laden soul can bear another's load, (Surah Bani-Israel: 15) 

B: That is, a person cannot keep a Roza or offer a Salaat (prayer) on behalf of another. Which means fasting or offering of Salaat by another person on behalf of the one responsible, doesn't drops the obligation liable on him/her. 

Allah (Subhanahu Wa Ta'ala) said: (Fast) a certain number of days; and (for) him who is sick among you, or on a journey, (the same) number of other days; (Surah Baqara: 184) 

B: Journey here signifies that the duration of someone's journeys shouldn't be less than three days. In this verse Allah (Subhanahu Wa Ta'ala) gave concession to the sick persons and the travellers that if fasting leads to enlargement of their sickness or poses threat to their lives or one faces hardship and pain during the journey then during the period of his/her sickness or travel, one can perform Iftar (break fast by consuming some food) and except the Ayyam Manhiya (Non-fasting days) one can perform the Qaza of the missed Roza's. Ayyam Manhiya are five, in which fasting is not permissible. That is the days of Eid's (celebrations) and 11th, 12th and 13th days of the holy month of Dhul Hujjah.

B: It is significant that regarding the Qaza Roza's of Ramadan it is mentioned in the Hidayah that the person willing to perform Qaza has got the choice to hold fast with continuity or at different periods. But it is Mustahab (commendable) to perform the qaza of Ramadan Roza's with continuity so that the obligation gets executed at earliest. If a person delayed so much in performing the qaza of them that the next Ramadan arrived, then it is necessary that such a person should perform the Fardh (mandatory) Roza's of Ramadan initially and should not hold any qaza Roza in this holy month and soon after the departing of it should perform the qaza of the previously missed Roza's and it is notable that, delayed performance of them doesn't makes Fidya (penalty) as Wajib (Compulsory). Only executing them is sufficient enough.