Sometimes When I fart it makes my pant wet and After rubbing my private part I found some dirt (not stole). Just like general dirt which can be got through rubbing.And Sometimes I see this kind of dirt(not stole) in the area near private part after farting. My question 1. Does the wet fart make my pant impure ? 2. With the perspective of question (1) is it obilige to change pant ? 2. Does this dirt (not stool) which I found through rubbing my private impure ? 3.Without rubbing the dirt I found near the area of private part after farting is impure or not ?
Yes because it is najis and najasa of the clothes you actually wear means that you cannot pray with these clothes.
Of course as you are not allowed to pray in a impure state: The purity you need is that of your body parts which can be achieved by ghusl or wudu' depending on the kind of impurity, that of your clothes, that of the place you pray on/at.
I'm not sure if I got you well, if there's dirt on your clothes then they are impure, if you didn't know they are your prayer might be considered as valid, but if you had time to repeat it you may be recommended to do so. IMO one can clearly feel a wet fart and therefore it should be clear that there's impurity.