I lost my friend' football, I should----- I shouldn't-----

i lost my friend's football, i should try my best to find it out./i shouldn't cover the truth.
lisa got a bad grade in P.E, she should get more exercises when she has spare time./she shouldn't ignore the fact that she's lacking of exercises.
mother wont let me go to kitty's birthday party, i should try to persuade her./i shouldn't insist that.
dave doesnt want to go the children's palace to learn piano, he should specify the reason why./he shouldn't force himself to do so.