Please explain about this sentence.　:'( "Attitudes towards learner errors have changed considerably in recent decades. Approaches based on behaviorist principles(particularly audiolingualism)" advocate the initial avoidance of errors and their diligent correction should they occur." I do not understand the part"their diligent correction should they occur". 1:What "their" and "they" stand for? I mean, whose diligent and who do occur? 2:Why does the "should" precede "they"? Thanks a lot.