4.37 O Arjuna, yatha, as; a samiddhah, blazing; agnih, fire, a well lighted fire; kurute, reduces; edhamsi, pieces of wood; bhasmasat, to ashes; tatha, similarly; jnanagnih, the fire of Knowledge-Knowledge itself being the fire; kurute, reduces; bhasmasat, to ashes; sarva-karmani, all actions, i.e. it renders them ineffective, for the fire of Knowledge itself cannot directly [Knowledge destroys ignorance, and thery the idea of agentship is eradicated. This in turn makes actions impossible.] burn actions to ashes, like pieces of wood. So, the idea implied is that full enlightenment is the cuase of making all actions impotent.
From the force the context [If the body were to die just with the dawn of Knowledge, imparting of Knowledge by enlightened persons would be impossible, and thus there would be no teacher to transmit Knowledge!] it follows that, since the result of actions owing to which the present body has been born has already become effective, therefore it gets eshausted only through experiencing it. Hence, Knowledge reduces to ashes only all those actions that were done (in this life) prior to the rise of Knowledge and that have not become effective, as also those performed along with (i.e. after the dawn of) Knowledge, and those that were done in the many past lives.
Since this is so, therefore,