It is so thoughtful of you to ask me, but unfortunately I have a rather important previous engagement on that day.

您对我的邀请真是想得周到。但不巧的是，那一天我预先已有重要的约会。

youdao

I'd love to, but I'm afraid I have a previous engagement.

我很宁愿答应往，但生怕已跟他人事前有约。

youdao

I cannot see you on Monday because I have a previous engagement.

星期一我不能和你见面，因为我有约在先了。

youdao