Re: Questions about Hadiths
Only two criteria are used. 1. Chain of narration is authenticated. If the chain of narration is authentic and complete then the Hadees is considered authentic. 2. Repetition , if same hadees is quoted by many narrators it is considered authentic.
Hadees were being recorded during the time of Holy Prophet(SAW) , first compilation of those ahadees took place first time after 200 years of his(SAW) death.
I do not have that figure but a substantial number of ahadees came from his(SAW) regular circle of close friends and family.
Somethings have a black and white answers in Quran and others do not a big example is payment of zakat . Where as it is mentioned in Quran that Muslims are supposed to pay zakat but how much are they supposed to pay , it comes from the Sunaah of Holy Prophet(SAW)
So those things which are not mentioned in Quran or which are not so black and white , but people need guidance for them then scholar look to other sources. Like hadees , sunnah , ijma , ijtehad , qyas etc.
These are simple answers of your simple questions. If you come up with more questions please do ask.