What is the ruling on the prayer of one who said "Subḥāna Rabbiya al-ʿAẓīm" in prostration (*sujūd*) and forgot to correct it?
All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah.
Saying “Subḥāna Rabbiya al-ʿAẓīm” in bowing (rukūʿ) and “Subḥāna Rabbiya al-Aʿlā” in prostration (sujūd) is one of the hayʾāt (non-essential sunnahs) of the prayer. Whoever omits it, whether deliberately or by forgetfulness, or replaces it with another remembrance (dhikr), his prayer is valid and he does not perform sujūd al-sahw. Shaykh al-Islām Imam al-Nawawī (may Allah have mercy on him) said: “Our school holds that one does not prostrate for omitting the audible or silent recitation, the tasbīḥ, and the rest of the hayʾāt.” [al-Majmūʿ, Vol.4/p.127]
If he performs sujūd al-sahw for something that does not call for it, his prayer is invalidated, because he introduced a prostration of forgetfulness without a cause, and he must repeat the prayer. The exception is one who did so out of forgetfulness or ignorance, whether because he recently entered Islam or because he lives far from scholars; he is excused and his prayer is not invalidated.
It is stated in Mughni al-Muḥtāj (Vol.1/p.417): “It is stated in al-Anwār: If he does something that does not call for sujūd al-sahw, thinking that it does, and then prostrates, his prayer is not invalidated if he is ignorant because he recently entered Islam or because he is far from scholars.”
Since this issue is hidden from the laity, as it is among the subtleties of knowledge, and the laity are excused for ignorance of whatever is of such a nature, and their act of obedience is not invalidated because of it, it may be said that the prayer of one who has not acquired knowledge of the issue is not invalidated. As for after knowing it, the prayer is invalidated.
Whoever realizes while still in the prayer that his prostration was out of place, it is sunnah for him to perform sujūd al-sahw. And Allah Almighty knows best.
When is a child instructed to fast?
A child is instructed to fast at the age of seven if they are capable of fasting and have reached the age of discernment, by analogy to prayer. They should be encouraged but not forced, so they can become accustomed to it. It is obligatory for their guardian, whether a father or another, to instruct them.
What is the ruling on making up missed fasts from many years?
A Muslim must make up for any missed fasts, as they are a debt owed to Allah. The Prophet ﷺ said:
"The debt owed to Allah is more deserving of being fulfilled." [Narrated by Muslim]
Whoever has missed fasts must make them up as long as they are alive and capable of fasting.
Someone wiped over his shoes after wearing them in a state of purity, then took them off and prayed without them?
Ablution is not invalidated by taking off leather socks or shoes after wiping over them. However, whoever takes them off after wiping must wash his feet only. If he prayed without washing his feet, he must wash his feet and repeat the prayer. And Allah the Almighty knows best.