Well there is a past subjunctive. It is formed exactly as the remote past but the "a" that constitutes the ending of the concord is pronounced as a much shorter vowel

The negative is formed in one of two ways: by prefixing a- to the positive form, or by inserting -nga- after the concord. In either case the ending is -a not -i.

I am pretty hazy still on where this tense should be used, I'm not worried too much for now, all will become clear in time no doubt.

Maybe our mother tongue speaking members can shed some light?