past tense formation

In my new conlang, Denanan, I have something of a problem.

Denanan is descended from Dweinasen (like Rhêndrin, in fact it's Rhêndrin's
western neighbor) and in Dweinasen the past tense is differentiated from the
present only by a change of final vowel: aninka "you give", aninko "you
gave". In Denanan a, e and o coalesce into a, so they both become "aninka".
I've sorted out a pretty basic way of forming the past tense for transitive
sentances (in fact I stole it from Hindi), I put the subject in the
instrumental, the object in the nominative and use the past participle:

Karyan cas'alá makar cas'alinta.
carpenter-nom story-acc boy-dat tell-pres-3s.
The carpenter tells the story to the boy.

Karyano cas'al makar cas'alayan.
carpenter-inst story-nom boy-dat tell-ppt.
The carpenter told the story to the boy.
-or-
By the carpenter the story to the boy was told.

s' stands for s-acute, by the way, pronounced either /S/ or /ç/, depending
on the dialect.

This is fine for transitive phrases, but what about intransitive?

S'annah anninta.
bread-nom burn-pres-3s
The bread burns.

But in the past:

S'annah annayan.
Bread-nom burn-ppt
The bread burnt.

which doesn't sound right. Not only that, certain verbs are intransative yet
take an object (I know, this is confusing). For instance, dali "to go" does
not take an accusative object, it takes the preposition ya and the dative:

Yañanay ya tálanar dalita.
man-pl-nom to mountain-dat go-pres-3p.
The men go to the mountain.

-but-

Yañanayo tálan dalayan.
man-pl-inst mountain-nom go-ppt.
The mountain by the men was gone.

It makes no sense. Any suggestions? How are such difficultie resolved in
similar natlangs and conlangs? And have I unwittingly created an ergative
system (I remember someone saying that Hindi is an ergative language in the
past tense) .

Dan



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E souvein-te della veritát que se ja dissó,
                 And remember the truth that once was spoken,

Amer un autre es veder le visaic de Deu.
        To love anonther person is to see the face of god.
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