如果你决定了什么时候来，请告诉我。怎么说？

Please tell me if you have decided when you will come.

看是口语还是邮件

参考说法：

口语：Please let me know on which date you decide to come.
书面：Please inform me on which date you decide to come.

please tell me if you decided when you will come.

个人觉得这样说比较恰当也.

If you decide to come,let me know when

once you decide when you will come, please tell me

Tell me if you have decided.