What is the ruling on purifying shares for past and coming years, and what is the equation for purifying shares?

It is permissible to trade in pure shares and forbidden to trade in prohibited and mixed shares. A pure share is the share of a company with permissible activities that does not deal with usury, whether by depositing or borrowing. Pure shares may involve some impermissible elements, such as a small proportion of trading in mixed shares or commercial insurance returns. It is obligatory to purify what is certain or highly probable to be money derived from an impermissible source. If the shares were pure and you were unaware of the obligation of purification, then purification for past years is not required, based on the Almighty's saying: "So whoever receives an admonition from his Lord and desists, then what has passed is for him." Purification is required annually by paying the required percentage once.