Klingon does not have tense (past/present/future), but it does have aspect (completed/ongoing/neither).
This sentence has the verb chaghpu' with the aspect suffix -pu' which indicates that you are talking about a completed action.
In this course, we equate that to English perfect tenses such as present perfect (has dropped), past perfect (had dropped), or future perfect (will have dropped) -- you can see that while those are different tenses, they all have the same aspect, i.e. perfect.
While Klingon perfective is not exactly the same as English perfect, in this introductory course we equate them as English does not have anything that is exactly like the Klingon perfective aspect.
In English, we usually mix up tense and aspect and call things such as present continuous or past perfect separate tenses -- Klingon does not have the time part of tenses but does have aspect.
mizinamo has explained the policy of this course, but the fact is your translation is just fine. chaghpu' refers to completed dropping; chagh refers to non-completed dropping. When you don't use an aspect suffix, the verb is explicitly not that aspect. The difference between chagh and chaghpu' when dealing with the past is nearly impossible to show in English. In nearly all cases, both simple and perfect tenses should be accepted.
Marc Okrand himself often uses English present perfect tense to translate Klingon perfective. The policy of this course to only accept English perfect tenses in translation is wrong.