we've been studying couplets in my english class.. & we need to analyze specific ones.. hmm.. i dont even know if im posting this in the right place (sorry if im wrong

) but i really dont get one of the couplets i have to analyze! can neone help me?
"but when to mischief mortals bend their will,
how soon they find fit instruments of ill"
i dunno if my brain is just fried from the other couplets i had to analyze.. but im stuck on this one!