✔ 最佳答案
The verb (bare infinitive/ verb without 'to') after 'wish' should be in the simple past: knew
I just wish I knew the truth behind the lies. (Correct)
It means that you do not know (but wish that you knew/ but you can't change the fact). It is a grammar rule for the use of 'wish' for this situation.
An example: I wish I were in Canada with my family. (but the fact is, you are here and not in Canada and you can't change this fact)
'just' is an adverb to modify 修饰 'wish". 'just' does not bear any rules for the verb after 'wish'.
I hope I have helped you!