他答应帮忙用英文怎么说

"He promised to help" 在英语中可以这样表达。

"Promise" 是英语中的动词，意思是 "to give one's word that something will happen or be the case"，即 "答应，保证"。

例句：

He promised to finish the work by the deadpne. 他答应在最后期限前完成工作。
She promised to attend the meeting. 她答应参加会议。
I promise to be there on time. 我保证准时到达。

另外，"to help" 是英语中的不及物动词，意思是 "to make it easier for someone to do something by offering one's assistance"，即 "帮助，提供帮助"。

例句：

Can you help me with this heavy box? 你能帮我搬这个沉用的箱子吗？
She offered to help with the cooking. 她主动提出帮忙做饭。
I'm here to help in any way I can. 我来帮忙，无论怎么帮都行。

希望这些信息对你有帮助！