"Slowly, the race finished."
Translation:Lentamente terminó la carrera.
That's not correct. It means gently and softly AS WELL AS slowly. But if you look at the examples tab on the dictionary page linked below you will see that it is mostly used for slowly. The examples are not written by SpanishDict, they are blindly collected through internet searches.
Right. However, 'slowly' is often implied within the meaning of 'gently' and the word 'despacio' (which DroppedBass specifically mentioned) can mean 'gently'... hence: not wrong. I agree that they aren't always synonyms. But they certainly can be... in either language.
The imperfect (terminaba) is not applicable. It doesn't really matter that the race finished slowly.
A common use of the imperfect is to set the scene, so if you used terminaba, then it's something like "Slowly, the race was finishing", which feels incomplete, and would have the reader asking "and then what?"
Lentamente, terminaba la carrera, cuando se cayó mi abuela.
You changed out of passive voice in the present perfect to active voice present perfect. The perfect tenses are the most common uses of the past participle. They don't function as adjectives. Many predicate adjectives do resemble the past participle, but it is important to recognize them as adjectives because they have to agree with the gender of the subject. Verb forms never change gender.
"acabar" is a little different. It does mean "to finish/end", but carries with it a sense of the instant in which something finished.
Including "lentamente" in the sentence means we are talking about the final moments of the race, which perhaps dragged on for some minutes.
So using "acabar" (which relates to the instant) with "lentamente" (which relates to an extended period) just doesn't work.
If we were talking about something more instantaneous, then "acabar" becomes more useful. E.g. "La carrera acabó mal" = "The race ended badly"
I am not sure whether I agree with Duo's not accepting despacio, but there's definitely something there. I often Google phrases like despacio vs lentamente, which generally yield many results from discussions and sometimes even lessons about the difference. Here are a couple of the hits
Of course the "feel" of a word cannot be effectively taught by most systems. It just takes a lot of exposure to live conversation. But you will understand better if you react like I do when Duo shows a translation like She obtained milk. The issue is the connotations and subtext not the denotations. But just like there are few true, complete synonyms in any language, many times one language has a shade of meaning that cannot be directly translated to a single word in the other language. And that goes both ways with different words.
As I said, I am not sure that I agree with Duo excluding despacio due to the apparent subtlety of the distinction at a beginning or intermediate level. But I do think that it has effectively alerted you to the issue and you will be noticing when and where each word is used by native speakers which is the best they can do to teach the distinction. When all is said and done, beyond any regional, age related or social set variations of usage, some of it is still subjective.
When you are talking about the specifics of an exercise in the discussions it is difficult for users to respond as there are many variations of these questions. But if you are talking about a choose all the correct answer exercise then what I can tell you is that the errors in the ones that are incorrect are often not what you are looking at. They are often just using the wrong preposition (maybe a instead of de) or add irrelevant words like circa. .