同意句转换：My cousin said that he will decide to study hard.

决定做某事 : decide to do sth. | determine to do | decide to do
决定不做某事 : decide not to do sth | Decided not to do something
下定决定做某事 : make up one’s mind

make up one’s mind

make up his mind
下定决心

My cousin said that he will make up his mind to study hard.

My cousin said that he will (make) (up) (his) (mind)to study hard.