Translation:I have made a mistake.
I make mistakes in colloquial English doesn't mean I continuously make mistakes; it means that the speaker is not perfect. I am making mistakes could be interpreted as continually, or at least, occasionally, erring, while I always make mistakes or I constantly make mistakes would be interpreted as one's habit, tendency, or potential to make mistakes without reference to any particular mistake.
jIQagh is indeed sufficient for I make mistakes, but it doesn't specify whether you've made one or more errors.
You might make the distinction by being more specific about how you err. rut jIQagh Sometimes I err; pIj jIQagh I often err; reH jIQagh I always err. Or if I make mistakes is meant to indicate your potential to make mistakes, you might use jIQaghlaH I can err.
This is one of those instances where the grammar of English and Klingon don't match, so however you translate it will have a slightly different nuance than you started with.
Thanks for the explanations! I realized after I hit submit that -mey is used for nouns. I guess I wasn't sure how to pluralize Qagh while also indicating past tense. How would you say "I have made mistakes"?
Same answer: Whether there was one mistake or many is not specified in Qagh. jIQaghpu' can mean I have made a mistake, I have made mistakes, I made a mistake, I made mistakes, I will have made a mistake, I will have made mistakes, I had made a mistake, or I had made mistakes.
Think of the word Qagh as meaning err and you'll get it.
Would "I erred" not be an acceptable translation? "To err" has worked for all the other uses of Qagh so far, except for this one.
"I have erred" does work, so I guess it's the English/Klingon tense mismatch at work.
In other words, they're going to teach you wrong Klingon for the sake of simplicity.
Make no mistake: I erred is a correct translation of jIQaghpu', whether Duolingo says so or not.
And just to be clear, I disagree that it is the best translation and that Duolingo is teaching wrong Klingon. There is a huge amount of overlap and we are not teaching the edge cases. We can't teach everything at once and have decided to leave those edges for you to learn after completing the course. I believe David disagrees about the specific areas and amounts of overlap. I suggest you consider his advice, but with it with a grain of salt, just as you should mine.