"Duras was a dishonored p'tak."
Translation:petaQ quvHa' ghaH DuraS'e'.
It's grammatically required (in standard Klingon) for the subject of a "NOUN is NOUN" sentence to have the -'e' suffix; it's simply a grammatical rule.
torgh 'oH pongwIj'e'. petaQ ghaH DuraS'e'. tlhInganpu' chaH SuvwI'pu''e'. etc. (My name is Torg. Duras is a p'tak. The warriors are Klingons.)
Similarly for "NOUN is in PLACE": nuqDaq 'oH vaS'a''e'? Qe'Daq chaHtaH tera'nganpu''e'. (Where is the Great Hall? The Terrans are in the restaurant.)
Up to now, I understood "quv" to be either a noun or a verb, but never an adjective. Is "quvHa'" here a past participle being employed as an adjective? I haven't had this yet in my lessons. If that is the case, then is it correct to say, for example, "SuvwI' HoH legh loD"? (The man saw the killed/slain warrior)
You are being introduced, here, to a grammar that has not yet been explained. It is explained in the Tips and Notes of the Adjectives unit which is just a little further in the course.
There are no adjectives in Klingon. Klingon does have a category of verbs which might be called "adjectival verbs". These can usually be recognized because they are defined as "be adjective" and use "am", "is", or "are" when in a sentence. For instance, quv wo'rIv. ("Worf is honored.") These adjectival verbs can also be used in a noun phrase in a way very similar to an English adjective (but having a reversed order from English adjectives). For instance, wo'rIv quv ("honored Worf").
quvHa' means "to be dishonored" and is just such an adjectival verb. You can say quvHa' petaQ ("The petaQ is dishonored.") or petaQ quvHa' ("the dishonored petaQ").
HoH is not an adjectival verb and can't be used in this way. There is a way to say it, though. You may have already learned (in the unit prior to this one) the verb suffix -bogh which allows you to use non-adjectival verbs in an adjectival way such as: Heghpu'bogh SuvwI' ("the warrior who has died") or even SuvwI' HoHpu'bogh ghaH ("the warrior whom he has killed").
Participle is not a good way to describe it. Klingon doesn't really have such a thing and it's just an adjectival verb being used adjectivally. That particular verb is defined with an English participle ("to be dishonored"), but that relies on how English does it, not how Klingon does it, most other Klingon adjectival verbs aren't defined with English participles, ex: Quch ("to be happy").