Question :

What should a worshiper who has forgotten a pillar of prayer do?

The Answer :

If he/she remembered the forgotten pillar before offering it in the following Rakah(unit of prayer), then he/she should offer it, and continue the prayer, then offer Sujood As-Sahw at the end of it. But, if he/she remembered that forgotten pillar after having offered it in the following Rakah, then he/she should offer a new Rakah in its(the one in which he forgot a pillar) place, and offer Sujood As-Sahw(Prostration of forgetfulness).