I will alway remember my high school Freshman English teacher Mr. Thurman who explained this to us, i.e. use the subject “who” in a sentence when one would use “he/she” whereas the object “whom” when one would use “him/her”.  Therefore, ” ‘Who’ taught you  this?”, i.e. ” ‘He’ (Mr. Thurman) taught us this.”, and “By ‘whom’ were you taught?”  “We were taught by ‘him’.”  Thanks Mr. Furman!

A cute review can be found here:  http://theoatmeal.com/comics/who_vs_whom