What is the error on the subject-verb agreement?

modern calculators, which can compute sums almost instantaneously, derives from abacuses, hand operated devices with beads that slide along rods.

when alex falls for the charming new boy at school, cole -- a handsome, funny, sports star who adores her - she can't believe she's finally found her soul who truly loves and understands her.  

at first, alex is blissfully happy. sure, cole seems a little jealous of her relationship with her close friend zack, but what guy would want his girlfriend spending all her time with another boy? as the months pass, though, alex can no longer ignore cole's small put-downs, pinches, or increasingly violent threats.  

as alex struggles to come to terms with the sweet boyfriend she fell in love with and the boyfriend whose "love" she no longer recognizes, she is forced to choose -- between her "true love" and herself.

explanation: