lieven:

Hope this helps a bit

Yes, it does ! And I think I understand (finally) what's going on.

If I walk in the kitchen to see that someone ate my pie, then I will
say: {QI'yaH ! chabwIj Sopta' vay' !}

But if I want to say that "when someone eats pizza, he always needs to
drink beer too", then I will say {pItSa' chab Soplu'taHvIS vaj reH HIq
tlhutlhnISlu' je}

Because I learned something new, I'm happy, however it amazes me, how
something so important eluded me so far..

Don't take this too far. It would also be perfectly correct to say chabwIj Soplu'ta' my pie has been eaten. -lu' is not only for hypothetical or subjunctive uses.

It's a matter of focus. -lu' takes the focus away from whoever would be the subject of the sentence, because it removes the subject entirely.

chabwIj Sopta' vay''e'
someone, definitely someone, has eaten my pie

chabwIj Soplu'ta'
my pie has been eaten

In the former sentence, I'm focusing very strongly on the culprit. In the latter, I'm focusing on my pie and the fact of its having been eaten; I'm not considering who has done it.

Use -lu' when you want to de-emphasize the role of the subject in the sentence, or when you just don't know or don't care who or what the subject is. Use vay' when you want to talk about someone or something and have them play an active role in the sentence.

-- 
SuStel
http://trimboli.name