18.41-60 Brahmana - etc. upto avasopitat.  Surely the intrinsic nature of the Brahmanas etc., does not voilate what has been  difined (above) by way of classifying their duties.  Therefore, as far as you are concerned, you have the intrinsic ality of the Ksatriya (warrior), and your nature i.e., intrinsic ality, does, without fail, assume the part of the inciter of yourself, even though you don't like it.  For, a person who acts simply being incited by that (natural condition), there is the strong bondage of the merit or demerit. Therefore, perform actions following the means of correct knowledge, taught by Me.  In that case, the bondage would disappear.  The intention of the principal sentence  (statement of the entire passage under study) is to help to get this idea.  The meaning of the subordinate sentences (statements) is evident.
 
Briefly  (verse 50) :  in short.  Knowledge : i.e. the one which has been explained earlier.  Nistha conveys, avoiding verbal jugglary, the meaning  'what has been determined'.  He who is endowed with intellect totally pure etc. :  All this has been almost explained already.  Hence, no more trouble is taken [to comment upon it].