Actually, the easiest way to remember it is probably just to think of
English verbs (such as "rise" and "raise", "sit" and sat", "lie" and
"lay", "drink" and "drench") that work the same way. Here's some
more. The strong verbs are on the left; weak ones with causative and
transitive meanings are on the right:

STRONG ............... WEAK

drífa "to drift, rush, throng" : dreifa "to scatter, strew, sprinkle"
fara "to go" : foera "to bring"
liggja "to lie" : leggja "to lay"
rísa "to rise" : reisa "to raise"
sitja "to sit" : setja "to set"
sofa "to sleep" : svefja "to soothe, lull to sleep"
spretta "to spring up" : spretta "to make spring up, tear open, undo"
springa "to spring; spurt out" : sprengja "to cause to burst"
støkkva "to jump" : støkkva "to make jump, startle, drive off; sprinkle"
vinda "to wind, twist" : venda "to wend, turn; change, convert"

--- In, "Fred and Grace Hatton"
<hatton@...> wrote:
> In my bumbling fashion, I had never noticed the way those verbs
worked in
> the strong/weak meaning-altering way.
> Thanks!
> Grace
> > 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'.
> LN
> Fred and Grace Hatton
> Hawley Pa