"We are returning to Earth on a ship."
Translation:DujDaq tera' wIcheghlI'.
In Klingon, when you chegh a thing, you're returning to that thing. It doesn't matter whether English uses a to or not, the object of chegh is the location the subject returns to.
Strictly speaking, you ARE allowed to put the locative suffix on the object of a verb like this. Klingons consider it overkill. Said this way, the sentence would be DujDaq tera'Daq wIcheghlI'. Notice the prefix wI- means tera'Daq is still the object of the verb. Without an object, the destination of such a verb remains unstated, no matter how many locative nouns you have dangling at the front.
-vaD is not appropriate here. You're not returning for the benefit of Earth, and Earth isn't receiving the result of your returning as an indirect object.
Notwithstanding the use of the word on, I wonder about using the locative -Daq in this sentence. The on here is being used more instrumentally than locatively. Query whether something like tera' wIcheghmeH Duj wIlIghtaH/wIlo'taH ("To return to Earth, we are riding/using a ship") would convey the sentence more accurately.
The usage is correct. For verbs who give their objects an inherent locative sense, any other locative you put in the sentence can't refer to what the object refers to. Okrand specifically addresses this with verbs of motion. If you say yuQ vIjaH, it means you go to the planet, but if you say yuQDaq jIjaH, it means you are on the planet as you go (somewhere).
chegh is a verb of motion, with a locative object, so it follows the same rules. tera' wIcheghlI' We are returning to Earth. A non-object locative on chegh refers to the place that the returning is taking place, not the destination of the returning. DujDaq tera' wIcheghlI' We are returning to Earth on the ship. The act of returning happens on board a ship.
So the sentence literally means something like We are aboard the ship as it makes its way back to Earth. Earth is our destination; the ship is where we do the returning.
Interesting...in languages that have both instrumental and locative cases (like Polish), I think you would use the instrumental, not the locative, to say "I am returning to such-and-such a place on such-and-such a mode of transport".
Compare (courtesy of Google Translate) "I am on the train" ("Jestem w pociągu" -- locative form of "train") with "I am returning to Warsaw on the train" ("Wracam pociągiem do Warszawy." -- instrumental form of "train").
Anyway...obviously Klingon is not Polish, and Klingon does not have an instrumental noun suffix. I don't question that -Daq is correct in the sentence, but it illustrates that -Daq can communicate not just where something happens, but also how.