conditionnel passé vs. past perfect

Question

I'm confused because on many other sites I see both these used as conditionnel. E.g., J'avais pu = I had been able to/could have
Si tu avais pu, tu aurais fait = If you could have, you would have.
Please explain.

Accepted Answer

Ruth, 
The conditional mood is not used in the ‘si clause’ of a conditional statement. 
The first French example you use is in plus-que-parfait, and is not a conditional statement - there is no if/then (no si clause followed by ‘results’ clause). 
‘J’avais pu…. ‘ is not in the conditional mood in French, that would be ‘ j’aurais pu….’
Without full context, cannot confirm whether and how the translation of ‘j’avais pu …’ as ‘could have …’ fits, but if it does, this probably represents the difficulties arising from the different uses of ‘could’ in English as a past tense or conditional. (Similar difficulties arise with ‘would’)
The 2nd example is an ‘impossible’ conditional - the situation posited is in the past and did not happen. The if (si) clause is in plus-que-parfait, and the then (result) clause in conditional perfect. 
See links below for use of ‘past conditional’ in hypothetical or conditional statements.
 https://www.lawlessfrench.com/grammar/si-clauses-third-conditional/
 https://www.lawlessfrench.com/grammar/conditional-perfect/