I would have helped you if I [knew] about your trouble.

A third-conditional sentence needs the past perfect in the 'if' clause to match 'would have helped'. So 'had known' is correct. The simple past 'knew' belongs to a second conditional and fits present or unreal situations, not a missed past opportunity. Match the tense in the if-clause with the main clause: past perfect with would have, simple past with would, and simple present with will.