Mostly it needs a blunder (but these are quite easy to make) or the endgame needs to arrive so that White on the move move can immediately take the knight without a recapture, or skewer or fork king and knight with the bishop winning the knight. About 10% of White to play positions where he cannot immediately capture and Black's king and knight are not on the same diagonal (hence cannot be both attacked by the bishop - but this also drops many positions where that is true anyway) are theoretically won by White.
The above type of position can take up to 106 moves to win against accurate defence, e.g.:
(Note the position after White's move 67 is clearly won for White.)
It can be difficult to win against an EGTB even from a won position. In competition play the ending may be theoretically won but impossible to win under the 50 move rule (as in the above example) but from 1st. of July this year the 50 move rule applies only in competition rules.
hi. i had bishop and knight. he had knight. i continued to play and eventually he exchanged knights. can i capture the knight with bishop and knight v. knight? or only if he blunders?