Klingon doesn't have a passive voice..

instead, it has the indefinite subject suffix {-lu'}, which means
"someone/something unspecified does/is".

Now..

True, one *could* translate the sentence {vIghro' leghlu'} as "the cat
is seen", but the exact/literal translation of this sentence, is
"someone/something unspecified sees the cat".

The cat is seen is an "exact/literal translation." So is someone/something unspecified sees the cat. When translating between languages that don't share similar grammar, you ignore the grammar of the original and just translate the concepts.

When we go out of our way to say someone/something unspecified sees the cat, what we're doing is illustrating how the grammar of the Klingon means what it means by casting the English is a more similar form.

Klingon, not only lacks a passive voice, but it lacks tenses too.

Saying {yotwI' Suv 'avwI'} can mean any of the following:

the guard fights the intruder
the guard fought the intruder
the guard will fight the intruder

All these alternate translations are valid, and the thing which will
define which of them is meant by the author, depends either on
context, or on the presence of time stamps.

But lets say, I write:

{wa'Hu', yotwI' Suv 'avwI'}

What does it mean ?

Some time ago, when I'd see it, without hesitation, I'd translate it
as "yesterday, the guard fought the intruder".

But recently I realized, that as in the case of the {-lu'} the literal
translation isn't passive voice, here too, the literal translation is
different..

It's not "yesterday, the guard fought the intruder"; it's rather
"yesterday, the guard fights the intruder".

Now, perhaps someone may wonder: "is this distinction important, and
if yes then why ?".

I think, that It's *very* important, because once someone begins to
think in the literal meaning of what the grammar of the sentence in
question actually says, then he'll be better able to use aspect
properly, depending on what he actually wants to say.

Lets see an example:

{wa'Hu', yotwI' Suvmo' 'avwI', QeHchoH HoD} = Yesterday, because the
guard fights the intruder, the captain becomes angry. (the two events
happen simultaneously)

{wa'Hu', yotwI' Suvpu'mo' 'avwI', QeHchoHpu' HoD} = Yesterday, because
the guard has fought the intruder, the captain has become angry. (the
event of the guard fighting the intruder has been completed, the
captain became angry sometime either during the fight or after it had
been completed, and the event of the becoming of the captain has been
also completed)

{wa'Hu', yotwI' Suvpu'mo' 'avwI', QeHchoH HoD} = Yesterday, because
the guard has fought the intruder, the captain becomes angry. (the
event of the guard fighting the intruder has been completed, and then
the captain becomes angry because of that event)

{wa'Hu', yotwI' Suvmo' 'avwI', QeHchoHpu' HoD} = Yesterday, because
the guard fights the intruder, the captain has become angry. (the
guard fights the intruder, without their fight having being completed,
but the event of the becoming angry of the captain is completed)

.. As is seen from these examples, there are variations between the
meaning of each sentence, which variations can only be perceived if
someone understands *exactly* what his eyes are actually reading,
without assigning the meaning of past tense to a verb, just because
that verb is being preceded by a {wa'Hu'}.

Or the meaning of future tense, just because the verb is being
preceded by a {wa'leS}.

I'm not sure I follow what you're saying.

Let me start with something a little simpler. wa'Hu' yotwI' Suvpu' 'avwI' Yesterday the guard fought the invader. Here I am, today, telling you how this event happened and was completed yesterday. The -pu' is needed because I want to describe an event as a complete whole: the fight happened and was finished and here we are, today, talking about it.

If I drop the -pu', I remove this completed aspect of the meaning. wa'Hu' yotwI' Suv 'avwI'. Here I am today, telling you about fighting that occurred yesterday, but I describe it as being something that didn't have a finishing point. Not finishing could mean a bunch of different things. I might be setting the viewpoint of the sentence in the middle of the fight yesterday. ("So there I am yesterday, standing in the middle of an invasion. I call for help. A guard shows up. One of the invaders attacks him. The guard fights the invader. The invader falls for a feint, and the guard kills him." The viewpoint of the story is advancing along with the narrative. Each sentence is the current moment as it happens.) I might be describing a general fact that isn't an actual event. ("We spent all day yesterday fighting our respective enemies, retreating, and going back for more. I was fighting a sabre bear. The captain was fighting energy beings. The guard was fighting an invader.")

{wa'Hu', yotwI' Suvmo' 'avwI', QeHchoH HoD} = Yesterday, because the
guard fights the intruder, the captain becomes angry. (the two events
happen simultaneously)

They might be simultaneous. But the important thing is that they're not complete or continuous. The sentence might be putting the listener in the viewpoint of the action as it's happening. Or it might be talking about how it's the guard's job to fight invaders, and because of that, the captain becomes angry. (yotwI' Suvmo' 'avwI' because the guard fights intruders may refer to the guard's job to fight intruders rather than a particular fight with a particular intruder.)

What it doesn't mean is "Remember yesterday, when the guard fought the invader? The sure made the captain angry!" Those actions (fought, made angry) are being described from a viewpoint of today, where they are already completed.

{wa'Hu',
      yotwI' Suvpu'mo' 'avwI', QeHchoHpu' HoD} = Yesterday, because
      the guard has fought the intruder, the captain has become angry.
      (the
      event of the guard fighting the intruder has been completed, the
      captain became angry sometime either during the fight or after it
      had
      been completed, and the event of the becoming of the captain has
      been
      also completed)

The presence of -pu' does not order the events described. Ordering events, saying something comes before or after something else, is tense. This sentence is looking back on the events of yesterday and telling us that the fighting is completed and the getting angry is completed. It is merely implicit that the fighting started before the getting angry could occur, since the one caused the other. But it is not necessarily the case that the fighting was over before the anger was begun.

{wa'Hu', yotwI' Suvpu'mo' 'avwI', QeHchoH HoD} = Yesterday, because
the guard has fought the intruder, the captain becomes angry. (the
event of the guard fighting the intruder has been completed, and then
the captain becomes angry because of that event)

The order is correct, but you have missed the full implications. First of all, the wa'Hu' seems to be attached to the subordinate clause rather than the main clause, so this might be interpreted as [wa'Hu' yotwI' Suvpu'mo' 'avwI'], QeHchoH HoD. This would mean that we're here today, looking back on the events of yesterday, and describing the fight that occurred and was completed. The captain is with us now, and gets angry in this moment while reflecting upon this fight.

Another interpretation you may have meant would be something like [wa'Hu' QeHchoH HoD], yotwI' Suvpu'mo' 'avwI'. We're here today, but the speaker is setting us into the viewpoint of something that happened yesterday, putting us into that moment. At the moment, the captain is getting angry. He or she is getting angry about the fact that the guard fought the invader. We don't know when the guard fought the invader, only that it occurred and was completed sometime before the captain gets angry. But this "before" is due to our understanding of cause and effect, not due to any tense markings or time stamps.

-pu' does NOT NOT NOT mean "before the given time context." That would be tense. -pu' means "described as a completed event."

-- 
SuStel
http://trimboli.name