"Are you Klingons? Yes, we are Klingons!"
Translation:tlhIngan tlhIH'a'? HISlaH, tlhIngan maH!
Please have another look at the tips and notes for the "Basic" unit: https://www.duolingo.com/skill/kl/Sentences-1/tips-and-notes
In particular, the section labelled "Plurals" and especially the last paragraph in that section.
(I trust you have been reading the tips and notes regularly? If not -- or if you did not even know they existed -- let me know and I'll tell you how to find them.)
In a sentence with a normal verb the topic marker is completely optional and there is no pattern to find. However, in a sentence that uses a pronoun as the verb "to be" there is a very special rule and that seems to be where you are confused.
The pronoun in a sentence like this acts as both subject and verb and takes an object normally. So tlhIngan maH is a complete sentence of this type meaning, "we are Klingons." No -'e' is needed when it's just an object and a subject pronoun like this. Similarly with the question tlhIngan tlhIH'a'? There is no confusion about who maH refers to or who tlhIH refers to, so no clarification is needed.
The complication comes when I am using a third person subject. If I say, tlhIngan ghaH that is a perfectly grammatical sentence for "he/she is a Klingon." But now, unless context has indicated exactly who I am talking about, there is a wide selection of who the "he/she" might be referring to. If I want to be more specific, in English, I could say, "The captain is a Klingon." Here "the captain" is the subject. But when we try to say that in Klingon, we run into a problem. Do you remember how I said the pronoun (in this case ghaH) is also the subject? So where do I put "the captain" if the pronoun is already the subject?
There's a special rule for adding a specific subject onto this kind of sentence. You put it after the pronoun and mark it with -'e'. In this case it is not a normal topic marker and it is absolutely required. It is participating in a special rule that allows you to add a subject noun to a pronoun that is acting like the verb "to be". It will not allow you to add a duplicate subject or object anywhere else and it is required when adding a specific subject on a sentence which is using a pronoun as "to be".
In the sentence tlhIngan ghaH, the word ghaH is the subject. In the sentence tlhIngan ghaH HoD'e', the word HoD'e' is the subject.
You can illustrate the purpose of the required -'e' on pronoun sentences by speaking like Tarzan or Cookie Monster:
tlhIngan ghaH HoD'e'
As for captain, him Klingon.