when 引导条件状语从句讲述的是客观情形.
if引导的则是假设性的条件状语从句,通常遵循：“主将从现”,即主句用将来时,从句用一般现在时!
另外,两个单词的词义的不同,也能区分.假如原句中usually 改为will.则用if 较好.即If they see other students doing better than them,they will believe that those students have a higher IQ
and that there is nothing they can do to change facts.