SuStel:

But I can tell you that -lu' does not mean "someone does something to someone."
It means "someone or something unspecified or abstract does something." Maybe
it's done to something, maybe not.

This confuses me greatly;

I read again section 4.2.5. of tkd, and yes the first thing it says is
"This suffix is used to indicate that the subject is unknown,
indefinite, and/or general".

On the other hand though, immediately it continues by saying: "Since
the subject is always the same (that is, it is always unstated), the
pronominal prefixes (section 4.1.1) are used in a different way". And
proceeds by giving examples of the kind "someone/something does
something to someone/something".

Even the example {Soplu'} is given as "it is eaten". Of course it
could be also taken as "someone eats it"; but, how can someone write
{Soplu'} in order to say "someone eats (period)" ?

If I wrote Sop HoD, does it mean the captain eats or the captain eats it? It means both! You know that.

So why should it trouble you that Soplu' means both one eats and one eats it?

Perhaps you will answer by saying "even if you say someone eats
(period), then again *that* someone is eating something so we
essentially say the same thing".

ok.. perhaps I can see your point; but my problem is that the tkd does
not have a single example like {yItlu'} for "someone is walking", or
{neplu'} for "someone is lying" etc. So, how can we accept such a
usage of {-lu'} ?

How about quSDaq ba'lu''a' Is this seat taken? in the appendix?

SuStel:

The difference is that vay' indicates a definite "someone,"
while -lu' might be used when that someone is more abstract

I can't understand this either; how is {lu'} abstract and {vay'}
definite ? Both mean someone. Someone unknown/unspecified. I can't see
any difference between them.

-- 
SuStel
http://trimboli.name