"Earth is the Terran's homeworld."
Translation:tera'ngan juHqo' 'oH tera''e'.
That's what some native Klingons think as well -- there is a dialect where the -'e' is not required in "to be" sentences.
But in the standard dialect (ta' tlhIngan Hol "emperor's Klingon"), which is the Klingon we teach, the -'e' suffix (including the apostrophes!) is required in such sentences -- it's simply a grammar rule that you have to follow.
Notice that the apostrophe in the English is before the s. That means that "Terran" is singular noun in the English sentence and the s is making it possessive instead of plural. To be plural in the English, it would have to be "Terrans' homeworld", which is also a fine description of Earth and accepted in the reverse translations. Also accepted for translations of tera'ngan juHqo' in the reverse exercise is, "Terran homeworld", which uses "Terran" as an adjective, as well as "homeworld of the Terran" and "homeworld of the Terrans". In English, the grammar makes it explicit whether you are using the word as a singular noun, a plural noun, or as an adjective and in the English sentence given it is a singular noun. The Klingon sentence is less specific and could result in any of those English translations.
Most likely. tera' ngan would be "inhabitant of Earth" and tera'ngan is "Earthling" - that's a pretty small difference there. Perhaps once upon a time it was a noun suffix. Or perhaps over time when the word could be thought of as referring to a race and not just a location it lost the space between the words. Officially, it is not productive; learners can't create new -ngan words, they have to leave a space between the words. I think if you started using some location that we don't have a -ngan word for and leaving out the space, most speakers would understand it as a designation of race, though many of them would criticize you for trying to use a word that isn't official cannon.