Question

Abet…She was unwilling to abet him in the swindle he had planned. Why not it can be -in the swindle he planned-? Why we added "had" and without it sentence would be correct or not?

Answer 1

had planned

Explanation:

One of the rules for past perfect tense is it demonstrates a finished action before a second point in the past.

He had already planned the swindle, before she was unwilling to help. The past perfect tense here just indicates what had happened first.

So, I would say when using two verbs in the past tense, and you want to say one thing happened before the other...that would be a good application of the past perfect tense

So yes, I would use the past perfect tense. But if you were to use the simple past tense, I think the sentence would still sound ok.

https://www.perfect-english-grammar.com/past-perfect-use.html