> brenndi upp.
> (it) burned up (or he burned it up?).

"he burned [it] up". As often, the causative verb is weak (brenndi upp
"destroyed by fire"), the intransitive strong (brann upp "was destroyed
by fire").

It's the same with 'renna' strong = to run; 'renna' weak = to make run,
to cause to run, etc. 'søkkva' strong = to sink (to go under);
'søkkva' weak = to sink (cause to go under). And, with a shift in
meaning, 'drekka' (strong) "to drink"; 'drekkja' (weak) "to cause to
sink, cause to drown", and quite a few others.

I don't know if this helps to remember it--if not, ignore!--but the root
of the causative verb in these examples has the same vowel as the
preterite indicative singular + i-mutation: Proto-Norse *'brann' + the
causative infix 'j' and the infinitive ending 'an' = *'brannjan' became
Old Norse 'brenna'.