"We are not responsible."
Translation:Nous ne sommes pas responsables.
There is a slight difference between on and we:
On pouvait se marier en Russie en 1919 = One could marry in Russia in 1919. Imagine a history teacher is explaining this fact to his students.
Nous pouvions nous marier en Russie en 1919 = We could marry in Russia in 1919. Imagine a granny telling that to her daughter.
Why is the sentence structured that way though, couldn't just be "ne nous pas responsables"?
French negations work in two parts: ne... pas/plus/jamais/rien/personne/aucun...
"ne" is placed before the verb and "pas" just after: nous ne sommes pas responsables