必定会做英文翻译

can be done must be done应该用must而不是should,因为must是一定，必须的意思，而should是应该的意思。:)

Therefore, mother, I promise I will be a student who is all-round development of morality, intelligence physique and aesthetic.

be sure about something 做某事有把握Must be doing sth肯定要做某事

做某事有把握 (Doing sth. ) is under control. 肯定要做某事 (Sb.) is bound to do sth.

So, mom ,I promise that I will be a all-round develpment student