What is the ruling on the prayer of someone who reached the age of eighteen, was diligent in praying in the mosque, and showed signs of prostration, but did not know the meaning of purification until just before reaching this age? And is the hadith true that states that praying four rak'ahs on the last Friday of Ramadan, reciting Surat al-Kawthar and al-Qadr 15 times after al-Fatihah in each rak'ah, expiates 400 years of missed prayers, and for one's parents and relatives?

You, dear brother, and every Muslim, must strive to learn the rulings of Islamic law. It is obligatory upon you, according to the majority of scholars, to make up the prayers you performed without purification. If you are unable to ascertain those prayers with certainty, then you must investigate and make up what you predominantly believe will absolve your responsibility. This opinion is the most cautious and absolving. As for the hadith (definition) you mentioned, there is no doubt that it is a fabrication against the Prophet, peace and blessings be upon him.