Translation:You can come return this and get a refund within seven days.
"You can bring it back within seven days to get a refund." or, more faithfully, "You can come in, return it, and get a refund."
I think that the 来 emphasizes that one has to claim the refund in person.
I remember the 1990s when one had to first obtain a Return Authorization Number before shipping a deficient computer back to Dell.
The only reason why people return something is to get a refund or credit. Otherwise, you would just throw the item away. In English, we use "to" when the first action is needed to achieve the second action.
I have to buy a ticket (in order) to go see a movie.
You need to buy five items (in order) to get a discount.
I am returning this item (in order) to get a refund.
Why is it 'awful English'? Come is used a lot as a verb + bare infinitive. It acts almost like an auxiliary, You might not like it but millions use it in oral English, including professors when they tell students of office times https://forum.wordreference.com/threads/come-visit-come-to-visit.2015773/
if you hadn't noticed, Duolingo is an American company that tries hard to accommodate UK English, but UK English is not the standard. If you don't say that in your dialect, you'll have to just memorize that it needs to be there. It's a more faithful translation, and it is fine American English.
That's the trade off. They would have to oversimplify the English to make it work all the time through every level of a whole course to ESL levels. But there are too many languages for them to make Chinese courses in. It's definitely second-best learning a langauge via an imperfect third language.
On the bright side, the side effect is that both your Chinese and English should improve by having to do it this hard way.
I think the English implies that you can return it at any time but the refund comes within seven days. otherwise, it should say "You can come return this within seven days and get a refund." In other words, I think the "seven days" is for the "refund" not the "return." or, you can say, you can come back with seven days, return it, and get a refund.
@Richi - "keep losing" points? Come on, its a simple English sentence, not a whole essay. You may make a mistake once (I do that all the time), but take the question again at the end and clear it. Too difficult? When you are focusing on the English instead of the Chinese, it is kinda difficult, I guess.
Here's my simple view. If something is not grammatically very wrong with the English response; which changes the very meaning of the Chinese - e.g. "Teacher's vs Teachers" or "Working hard vs hardly working"; then it does not matter. We still learn Chinese, which is the goal. In fact we learn it better because we won't forget 来 now because of the "come".
And lastly, if we respond with any English sentence that substantially works as a translation of the given Chinese, how would anyone judge whether you got all the Chinese characters and the grammar - or are just guessing with a few keywords?
Anyway, that's what I think and this approach has helped me so far in learning multiple languages... Others can have their own approaches of course.
@Micah - "..because are illiterate.."? Really? The reality is that the best way to learn Chinese is to understand the Chinese. And that would, by definition, create what you call as 'Chinglish'. Else you are trying to convert Chinese into English - which is never going to help you learn the language.