What is the expiation for one who vowed to fast for a month but did not fast, even though he was able to, and can this fasting be compensated for by something else? And what is the expiation for one who vowed to pray for his entire life but did not adhere to it?

Fulfilling a vow of obedience is obligatory for the one who is able, based on the Almighty's saying: "And let them fulfill their vows," and the Prophet's (peace be upon him) saying: "Whoever vows to obey Allah, let him obey Him."

If the one who made the vow is continuously unable to fulfill it, then he must offer an expiation for a broken oath (kaffarat yamin), which is: feeding ten poor persons, or clothing them, or freeing a slave, or fasting three days if he cannot find anything.

If the intention behind "vowing to pray for an entire lifetime" was the obligatory prayers, then the vow is not valid. However, if the intention was supererogatory prayers, then vowing to perform what is difficult among them is disliked but must be fulfilled, and what is beyond one's capability is impermissible and requires expiation.