Past perfect + which tense?

I'm trying to say: He had also made her promise that when that day comes (->????), she leave and not look back.

I'm stuck about the right tense to use for the verb to come, since it's the "future in the past".

Thank you in advance for your help.

Solution 1:

You can regard this as a type of reported speech with her actual words being:

"(I promise that ) when that day comes I will leave and not look back."

When reporting speech it is customary to backshift the tenses. This results in:

She promised that when that day came she would leave and not look back.

Integrating this into the original sentence results in:

He had made her promise that when that day came she would leave and not look back.

Note that backshift is common even in reporting sentences about the future: "I'll call you tomorrow" > He said he'd call me tomorrow.

Here is an article about backshift (also known as the sequence of tenses): https://www.thoughtco.com/backshift-sequence-of-tense-rule-in-grammar-1689017