1) I forgot to repair (repair) my watch last weekend, so I had to use my father’s. 为什么?
2) I regretted making (make) such mistakes. My parents were angry with me. 为什么?
forget to do sth 的意思是“忘记做某事”,to do sth 所表示的动作没有发生过;forget doing sth 的意思是“忘记做过某事”,doing sth 所表示的动作已经发生过 。根据后面的句子 so i had to use my father's 可知道repair(修理)这个动作没有发生过,所以应该用 to repair;regret doing sth 的意思是“对做过某事感到后悔”, doing sth 所表示的动作已经发生过 。根据后面的句子 My parents were angry with me ”可知道 make 这个动作没有发生过,所以应该用 making。