if you say Я делал это it means I did it or even I was doing it
If you say я сделал это it means I have done it.
result and process
Соверешнный и несовершенный
Совершенный means actions have finished
несовершенный means actions is in a process
Я прощал ей всё, means she did bad stuff many times and every time I forgot her. it is a kind of process
Я просил тебя за это. means someone has done something bad and I have forgot him. it is a kind of result
Прости меня. means I want you to have forgot me for something. result. one time. It takes a moment.
Прощай ей её всё. means never mind if she does bad stuff. process. it might take long time, maybe even years