"He would have opened the door."
Translation:Él habría abierto la puerta.
Spanish is my first language, and I would never translate this sentence quite like that. I'd probably use "hubiera abierto". Then again, casual Spanish (especially in Latin America) doesn't always follow the rules rigidly, so I fully accept that my translation would probably be technically wrong.
It is very common, and accepted by the RAE to use the past subjunctive instead of the conditional in many cases. You will often see the past subjunctive on both sides of a conditional statement, for example, and this is considered correct.
Si tuviera más dinero, comprara un nuevo coche.
for Si tuviera más dinero, compraría un nuevo coche.
If I had more money, I would buy a new car.
When the past participle is being used as an adjective in a sentnece, then yes, the gender of the adjective should match the noun.
La puerta abierta = The open door.
However, the past participle is part of a compound verb in this sentence and not gendered.
So since there's no lesson for this, I went to: http://www.studyspanish.com/verbs/lessons/conditional.htm and now I'm wondering why I can't write "he would have opened the door" as "el abriria la puerta"