Re: Surah for Salat/Namaz
I searched for the answer to this part…and this is what i found that most satisfied me. Can We Recite the Same Surah in Each Rak`ah? - Islam Question & Answer
Will search for the answer to your second question too…but as far as I know…it is indeed a sin to forget what you learnt of the Quran…unless you are striving to review and learn for it and Still cannot…you are guilty and sinful, I, myself included in this sin, since i learnt the last 3 parahs of the Quarn in childhood and forgot many big surahs 
iA will have to learn all of them soon!!