I understand that if I wanted to say "When I was in London..." I would do so by saying "Nuair a bhí mé i Londain...", or to say "It was raining when we left..." I would say "Bhí sé ag cur báisti nuair a d'fhágamar..." Under what circumstances would I not use the a when using "nuair". In other words, what is the rule for translating "when" as a conjunction?

Go raibh míle maith agaibh.

Joe (The Inquisitive One)

Nuair a bhí mé i Londain.

Nuair nach raibh mé i Londain.

Nuair nár dhún mé an doras.

There's no other place where I wouldn't use a after nuair.

As for "It was raining when we left":

Bhí sé ag cur báistí agus muid ag fágáil.

would translate more as "It was raining as we were leaving".