19.5.5 nem que even if

Nem que belongs to the many compound conjunctions whose components suggest a meaning that they actually don't have. Nem corresponds to e não, nem can be substituted by e não. In construction like nem...nem, nem...não it has the meaning it suggests. (But as a conjunction, see below, the meaning is not even.)

nem ...nem / não ... nem / não... e não
O advogado não apresentou contestação, nem apresentará.
O advogado não apresentou contestação e não apresentará.
The lawyer didn't file an objection and won't do it.
Não como carne nem peixe.
Nem como carne nem peixe.
I eat neither meat nor fish.

In this case it is really a negation and the meaning is what the components suggest.

Furthermore nem is used before the indefinite pronouns todo, tudo and the adverbe sempre if they are negated. (not all, not always).

nem as negation particle
Nem tudo é complicado.
Not everything is difficult.
Nem sempre eu tenho vontade de ir ao cinema.
I am not always on the mood to go to the cinema.
Nem todos aqui falam alemão.
Not everybody here speaks german.

Nem can be used as well in other contexts as a negative particle, but the conjunction nem que is not a negation. Even if the event described in the subordinate clause is not realized, the event in the main clause will happen anyway.

Even if it rains, we go for a walk.

The event in the main clause will happen anyway, it is the exact opposite of negation.

nem que even if
Não estarias na minha vida nem que fosses o último homem no planeta.
You wouldn't be part of my life even if you were the last man on earth.
E não podia ter-te detido nem que quisesse.
I wouldn't have been able to stop you, even if I had tried.
Resolverei isto, nem que tenha de ser eu a fazê-lo.
I will resolve that, even if I have to do it myself.

The fact that the different components of a conjunction suggests a completely different meaning or, as in this case, the exact opposite, is a little bit confusing.

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