tera'ngan, in this case, is not referring to actual Terrans, but is being used as a classification for the type of foreheads being referrenced. tera'ngan Quch "a Terran forehead", tera'ngan QuchDu' "Terran foreheads". However, as you have noted, tera'ngan QuchDu' could also mean "the foreheads of the Terrans", though in this course we would be more likely to write that as tera'nganpu' QuchDu'. You seem to have the idea, but be ready for a curve ball every now and then.
Klingon doesn't have articles, and plural suffixes are usually optional, so the difference between "Terran foreheads," "Terrans' foreheads," "a Terran's foreheads" (maybe they're masks he keeps on a shelf), "the Terran's foreheads," and "the Terran foreheads" is not expressed in Klingon tera'ngan QuchDu'. The writer of the example may have had one of those in mind, but you shouldn't have to guess which one he was thinking of. tera'ngan QuchDu' can mean any or all of them at once. Since you have to pick one to write the English, any should be accepted.
When thinking only in Klingon, the isolated phrase tera'ngan QuchDu' only tells you that (1) you're talking about multiple or abstracted foreheads, and (2) the kind of foreheads you're talking about is associated in some way with one or more or abstracted Terrans. That's all you know. An "abstracted forehead" would be foreheads in general, not "a" forehead or "the" forehead.
What you DON'T know about the phrase is whether you're talking about any foreheads in particular and whether you're talking about any Terrans in particular. Without further context the Klingon simply doesn't express this.
All of those comments are interesting. Still this fact has not been mentioned: For years of his life, Worf was a "terran." So obviously, some terran foreheads ARE ridgy.
And in reference to your original (two-month old) question, in this language, much meaning is conveyed contextually. And there can be no better way to learn to understand (and be understood in) Klingonese, than by practical experience. A bar in Tijuana comes to my mind, as a fun setting to practice in.
Worf spent most of his late childhood on Gault, not Earth. It's unclear how long the Rozhenkos lived on Earth while Worf was with them, if he lived with them on Earth at all. He also presumably lived on Earth while a cadet.
The word tera'ngan is exactly analogous to the word Earthling. Try telling Worf he's an Earthling; he'll disagree. (In one episode, he even says, "I am not a man." He means, of course, that he is not a human. Tolkien used the word man like this when contrasting humans with elves, dwarves, hobbits, and so on.
Earthling might mean inhabitant of Earth, but it has a strong connotation of human. Very probably, the same is true of Klingon tera'ngan. Okrand has used the word a lot, and it has always been synonymous with human. Klingons talking to human tourists and businessmen in Okrand's language tapes always call them tera'ngan, not Human, even though the Klingons generally wouldn't have any idea of the human's home planet.
See my previous posts on this topic. tera'ngan means Earthling, but it also means inhabitant of Earth. The two are not synonymous. (Whether TKD uses Earther, Earthling, or Terran is immaterial. They mean the same thing.)
I can see you really, really want the be able to break apart ngan words literally. It doesn't work like that. A tlhIngan is not an inhabitant of tlhI. A verengan is not an inhabitant of vere. A verengan that lives on Deep Space Nine is still a verengan even while being a logh Hop tengchaH Hut ngan. Okrand's use of tera'ngan is consistent with humans being called tera'ngan even when the speaker has no idea if they're from Earth. A lemon is given to us as a tera' na'ran wIb, not a tera'ngan na'ran wIb. The evidence is stacked heavily against a pure inhabitant of meaning.
It's okay; languages are messy like that.
If it makes you feel any better, write the race name as tera'ngan but an inhabitant of Earth is a tera' ngan. That's not a canonical thing to do, but it would make your intentions clear to anyone reading your words.
Marc Okrand has weighed in with material on this question since we discussed it.
The element -ngan refers to a group of people with a particular identity, not necessarily to the inhabitants of a particular place. In general, a Klingon living on Earth cannot be called a tera'ngan. Only extreme hair-splitters would call all inhabitants of Earth tera'ngan, and they must endure the palpable eye-rolling of those around them.
This is now canonical.