'if-then' sentences using plus-que-parfait

Question

My question was asked below, but the answers provided did not address my confusion. One example given in this lesson is "Si nous l'avions choisi, nous l'aurions regretté".A previous lesson on 'if-then" sentences (Imperfect usage - hypothesis | French Grammar | Progress with Lawless French) says the following:  "You can never use Le Passé Composé nor other past tenses in this context." I took this to mean that all if-then sentences must use an "imparfait + conditionel présent" form. 
But here we are using the plus-que-parfait and the conditional passé in an if-then sentence. There is no mention of the plus-que-parfait + conditional passé form in the lesson linked to above.Can you please clarify the rules around using plus-que-parfait + conditional present in 'if-then' sentences? Thanks.

Cécile Janty-Davies · Accepted Answer

Bonjour Isabelle, 
To add to Maarten's answer, consider my response to Gregory, which students found useful. It illustrates the three cases of si-clauses -
https://french.kwiziq.com/questions/view/other-types-of-si-clauses
Also, we have taken out the Attention paragraph in the lesson body as it brought confusion, so thank you for bringing this to our attention.

Maarten Kamp · Answer

Isabelle, 
It seems that the lesson you are referring to is about the conditional in hypothetical or ‘ unlikely/improbable’ situations - if something were to happen, then something else would happen. ( The lesson in your link is not about the conditional specifically, but about plus-que parfait, with one example being a conditional sentence ).https://progress.lawlessfrench.com/revision/glossary/conjugation-tense-mood-aspect/english-present-conditionalHowever, the example you are questioning is about ‘ impossible ‘ situations - if something had happened in the past then something else would have happened, although neither event did. https://progress.lawlessfrench.com/revision/glossary/conjugation-tense-mood-aspect/english-perfect-conditional Below a couple of links on Laura Lawless site that might help further address your question regarding the French grammar usage. However, passé composé is not used in the ‘ si ‘ clause in either of these situations in French.
https://www.lawlessfrench.com/grammar/conditional-perfect/
 https://www.lawlessfrench.com/grammar/conditional-mood/