http://www.kentlaw.edu/academics/lrw/grinker/LwtaSubject_Verb_Agreement.htm
https://www.youtube.com/watch?v=KI6jX40WM3w
https://www.youtube.com/watch?v=37XBATSQm8U
https://www.youtube.com/watch?v=GbPckRd95u8
best video above