Confused if past or present tense is correct in this sentence

How about, "when they would make their marriage vows..."?

Thank you for your reply. I agree that does sound much better but I now have a repetition of the word would in quick succession! I'm getting myself in knots over this and maybe should rewrite this passage entirely.
I think after writing 57 chapters my brain is fatigued with it all !
Not thought so much about Grammar since I was at school and that was a long time ago ....

"She now knew that tomorrow, when they made their marriage vows in front of family and friends, it would all be part of an ultimate grand design." 

Something makes me think this might be okay as it is but I don't know the grammatical explanation. I agree that having the two "woulds" in quick succession is not ideal. Unless a better grammarian than I shows up it might be easiest to do a rewrite!

Thank you .
I'm glad you feel that it could be correct too as it makes me feel less stupid for writing it initially ! It's a bit of a conundrum as to why it sounds right when it could be totally wrong 😖

The sentence as it stands is wrong, because we're looking forward to an event that hasn't happened yet.

As written it should be make because she is talking about the moment the vows are made.

If you wrote "once they made" instead of "when they made" it would also be correct because she would be talking about the moment after the vows were made.

Hope this helps.

Thank you for your help.
I shall go with your suggestion of "once they made".
Thanks for clarifying how the difference in timelines changes things 👍

And it doesn't jar like when they make would even though it's correct.

That's the thing about grammar it changes, when something sounds wrong people stop saying it.

Glad I could help.

"She now knew that tomorrow, once when they made their marriage vows in front of family and friends, (it) would all be part of an ultimate grand design."

You don't need IT. Remove the commas and try again. "...tomorrow would all be part of..." You also don't need "all". "...tomorrow would be part of...

"She now knew that tomorrow, when they made their marriage vows in front of family and friends, it would all be part of an ultimate grand design."

Try

""She now knew that tomorrow's marriage vows in front of family and friends would be part of an ultimate grand design."

or

"She now knew that tomorrow's events would all be part of an ultimate grand design."

Your sentence is actually correct as it is. In narrative writing, especially in past tense, it is common to use the past tense form ("made") to describe events that are perceived from the perspective of a future moment in the story's timeline.

Let's break down why "made" is appropriate here:

1. Narrative Perspective: The sentence is describing the bride's thoughts about a future event from the perspective of the present or past moment in the story. In this context, "tomorrow" is referred to as a future point relative to the current narrative moment.

2. Past Tense Convention: When writing in the past tense, events that are in the future relative to the narrative present are often described using the past tense. This helps maintain consistency and clarity in the narrative timeline.

3. Grammatical Correctness: Using "made" here indicates that the act of making their marriage vows will occur in the future, from the perspective of the bride reflecting on it now. "Make" would imply present tense, which would not fit with the overall past tense narration of your novel.

Therefore, your sentence:

"She now knew that tomorrow, when they made their marriage vows in front of family and friends, it would all be part of an ultimate grand design."

is grammatically correct and appropriate for your narrative style (past tense, 3rd person POV). It effectively conveys the bride's contemplation of an upcoming event from a past or present narrative standpoint.