Nothing does. It could be either and both are accepted as correct answers. It could also be "will jump". Klingon does not add tense by changing the verb, it indicates the timeframe through context. So if we added the word wa'Hu' ("yesterday") then that would limit the answer to just the past tense "jumped".
English marks verbs for tense (when in time an action occurs) and aspect (how something happens over time). Simple past tense is regularly marked with -d or -ed. ("Simple" is the aspect; "past" is the tense.)
Klingon does not mark verbs for tense. Instead, it marks verbs only for aspect. Any Klingon verb might take place in the past, present, or future, and if you don't have any context to figure it out, you can't tell what is meant.
Klingon marks verbs for perfective aspect (whether something is completed) and continuous aspect (whether something is ongoing). A verb that is not marked as perfective or continuous represents a "simple" aspect: neither perfective nor continuous.
maSuppu' We jumped; we have jumped; we had jumped; we will have jumped. (-pu' = perfective)
Whenever the sentence is taking place, the jumping is already completed.
maSuptaH We are jumping; we were jumping; we will be jumping. (-taH = continuous)
Whenever the sentence is taking place, the jumping is ongoing.
maSup We jump; we jumped; we will jump
The sentence is either describing a general or permanent truth (e.g., "It was our job to jump every day"), or else whenever the sentence is taking place we're in the moment of a jump. The jump is not completed, and we're not concerned with what happens just before or just after that moment.
Note that there are two other aspect suffixes: -ta' (accomplished, means the same as -pu' but adds a connotation of having accomplished an action intentionally) and -lI' (progress, means the same as -taH but adds a connotation of there being a known stopping point). It is always allowed to use -pu' or -taH instead of -ta' or -lI', even if the action is intentional or has a known stopping point.
To tell WHEN a sentence happens, describe when it happens.
wa'Hu' maSuppu' Yesterday we jumped; yesterday we had jumped. Today I'm reporting to you an action that was completed yesterday. You wouldn't say wa'Hu' maSup to refer to an act of jumping completed yesterday, because using -pu' on a completed action is not optional.
wa'leS maSuptaH Tomorrow we will be jumping. Today I'm predicting that we will be involved in an ongoing act of jumping tomorrow.
DaH maSup We jump now. I'm describing what we're doing right now, or what we're about to do. It's not completed because we haven't finished doing it yet, and if we were jumping moments ago and will still be jumping moments later I am not interested in discussing that.
DaH maSuptaH We are jumping now would mean that we're jumping now, a few moments ago we were jumping, and in the next few moments we'll still be jumping.
Now for the bad news: this Duolingo course doesn't represent aspect properly. It incorrectly equates Klingon perfective with English perfect tenses (have done, had done, will have done), Klingon continuous with English progressive tenses (is doing, was doing, will be doing), and Klingon "simple" aspect with English simple tenses (do, did, will do). This equivalence is wrong. So if you see a simple past tense sentence in English and try to use an appropriate Klingon perfective to translate it (e.g., wa'Hu' maSuppu'), the course may mark you wrong. This is a design decision by the course creators to simplify the teaching process — they have sacrificed accuracy for ease of implementation. You'll have to grin and bear it, and remember how Klingon aspect really works.
This equivalence is wrong. So if you see a simple past tense sentence in English and try to use an appropriate Klingon perfective to translate it (e.g., wa'Hu' maSuppu'), the course may mark you wrong. This is a design decision by the course creators to simplify the teaching process — they have sacrificed accuracy for ease of implementation. You'll have to grin and bear it, and remember how Klingon aspect really works.
It might be worth noting that not all Klingon speakers agree with David on this. But I will admit that the equivalence is not perfect and there will be edge cases where a different interpretation may work better in translation. If you have questions about David's view of it or on how Duolingo does it differently from how David would do it, please feel free to ask.
The "not all Klingon speakers" come from a time in the old days when we taught that "perfective" meant "before the current time context" — which is actually a description of tense, not aspect. Tense tells when; aspect tells how. This incorrect idea was drilled into everyone, including myself... but a lot of canonical Klingon by Okrand himself contradicts this description.
The first contradiction I ever noticed was the sentence loSmaH ben jIboghpu' I was born forty years ago. If we went by the old description we used to use — that perfective aspect sets the action before the given time context — then this sentence would mean that as of forty years ago, I was already born. But that's not what the sentence means; it means that I was born forty years ago. It's just that I'm here, today, reporting the completed event after the fact.
I started questioning the conventional wisdom, and started finding evidence all over the place that Klingon perfective wasn't the same as English perfect tense. This led me to a wider study of aspect in Klingon.
You'll find that all of canon agrees with me, and much of canon contradicts the ideas used by Duolingo. You'll also find that what I say also agrees with linguistics in general. What Duolingo gets wrong isn't edge cases, it's fundamental ideas.
None the less, current speakers have also looked at the body of evidence and feel that the borders are not as strict as you insist. Much of what Duolingo does differently than you would is really thought to be acceptable either way, meaning that the sentences where there is really a significant difference are much fewer than you imply.
I don't insist on strict borders. That is a mischaracterization. I insist on understanding what things in The Klingon Dictionary mean. "Perfective" means something, and what it means is borne out in the sentences Okrand uses everywhere.
I also have not implied that Duolingo gets so many sentences wrong. It does get some wrong, but more importantly it allows wrong answers alongside right answers. It teaches, for instance, that Klingon simple aspect (no suffix) can be used for perfective ideas. That's what people learn when you tell them that maSup means We jumped. The vast majority of the time, someone will interpret that English sentence in the perfective sense, and now they've learned to ignore Klingon perfective unless the English is employing a perfect tense. It's not the sentences Duolingo gets wrong, it's the wrong lessons it teaches.
(Duolingo also teaches that Klingon continuous is equal to English progressive, even though they're used for different things. English present progressive (I am running) is used to describe a thing you're doing now, while English simple present (I run) is used for permanent facts or general truths. Klingon doesn't break things up this way; the unsuffixed Klingon aspect can be used for both permanent facts/general truths and describing what you're doing right now, while Klingon continuous can be used to describe what you're doing right now, but gives it a meaning of ongoingness. English present progressive can also have that sense of ongoingness, but there's no way to distinguish between the "what I'm doing now" and the "what is ongoing now" in English like there is in Klingon. These differences are important, and Duolingo largely fails to deal with them correctly.)