"Mara and Worf are Klingons."
Translation:tlhInganpu' chaH mara'e' wo'rIv'e' je.
It happens when a sentence is using a pronoun as if it were the verb "to be" AND an explicit subject (that the pronoun is already representing) is added on.
This sentence uses the pronoun chaH as if it was a verb meaning "they are". But we also explicitly state who the word "they" is representing, so we have to add -'e' to that explicit noun (or nouns in this case).
An explanation is given in the Tips & Notes, but I know it’s a lot of new information and some details are bound to be difficult to understand or slip through the memory. However, since Duolingo has hidden the Tips & Notes I want to make sure you know about them and where to find them. If you have not been reading the Tips & Notes, I would like to ask that you review those.
If you are doing the course on iOS or Android, you cannot currently access the Tips & Notes through the app. To access the Tips & Notes, you will have to access the course using a web browser at https://www.duolingo.com/. You can still do it on your mobile device, but you will have to use the web browser instead of the app (or you can do it from a computer). When you click on a Skill, it will expand to reveal a large Start button, a small key button, and a large Tips button (or a small light bulb button - the small light bulb button is pictured here).
If you click on the Tips button (or the light bulb if you have that instead) it will reveal the Tips & Notes and give you a detailed explanation of the grammar that is introduced in that Skill. If you have questions after reading the Tips & Notes for any Skills, we are happy to answer your questions, but many of your questions will probably already be answered in the Tips & Notes.