在英语中，当你答应某人做某事时，可以使用以下表达方式之一：

I promised him/her that I would do it.
我答应他/她我会做的。

I gave him/her my word that I would do it.
我向他/她保证我会做的。

I assured him/her that I would do it.
我向他/她保证我会做的。

这些表达都表示你已经答应了某人做某事，并且打算遵守诺言。

下面是一些例句，你可以参考一下：

I promised my friend that I would help him move next weekend.
我答应我的朋友下周末会帮他搬家。

I gave my boss my word that I would finish the project on time.
我向我的老板保证会按时完成项目。

I assured my parents that I would study hard for the exams.
我向我的父母保证会为考试努力学习。

希望以上回答能够帮到你。