改错Playing computer games cost the boy many time he should have spent the time doing his lessons

这个不止一个错误吧
Playing computer games costs the boy much time which should have spent doing his homework

把MANY改成MUCH即可

It costs the boy much time playing computer games, he should have spend the time to do his homework.

playing computer games costed the boy,because most of the time he should have spent doing his lessons

cost-takes should be has

貌似很少有人会用computer games这个说法，一般都是electricity games。。。。。