What is the ruling on failing to fulfill a stipulated vow, and is it considered a major sin, and what is the ruling on being unable to fulfill it, and what is the meaning of "out of faith and hope for reward" and the deeds that invalidate it?

Fasting Ramadan is an obligation that may not be abandoned except for a legitimate excuse. Fasting must be done with faith (iman) and anticipation of reward (ihtisab), without ostentation. Vowing to fast Ramadan is not binding because it is already an obligation. Whoever abandons fasting without an excuse, or fasts ostentatiously, is sinful, but no expiation (kaffarah) is required of him; rather, he must repent and make up the fasts. As for whoever is unable to fast, there is nothing upon him. "Faith (iman)" means believing in its obligation and in its reward, and "anticipation of reward (ihtisab)" means seeking reward from Allah and intending the Face of Allah, or having the resolve and desire for His reward without dislike or burden. What contradicts faith and anticipation of reward is a lack of sincerity, a lack of belief in its obligatoriness and its reward, and disliking the fast.