Re: Surah for Salat/Namaz
Yes it is a sin, if you forgot it after you became mature. Learning the Quran and then forgetting it is a big gunnah, thats why they say being a Hafiz is a big responsibility.
As for the question, you can repeat you Surah’s as much as you need to, but it is highly encouraged to learn atleast the surah’s of the 30th para. Imagin praying 20 rakaat’s in ramadhan and only have four Kul’s to repeat