Translation:I will collect your mail until you come back.
Because zurückkommen is a separable verb. Since bis here is a subordinate conjunction. It pushes the conjugated verb to the end. Now, that it's a separatable verb, the prefix goes between bis kommst. When this happens, it's reattached to the verb, hence zurückkommt. You have to do this.
See how in this sentence bis is done? „Kannst du nicht warten, bis wir an der Grenze sind?“
In this sentence totlachen is tot|lachen. „Der Igel war tief verletzt, weil der Hase sich fast totgelacht hat.“
I'm aware that 'post' in the singular is synonymous with 'mail', hence my only mentioning the plural- your original question.
When using 'post' to refer to mail, it's an uncountable noun- this means that it cannot be pluralised to 'posts'.
Also, the German sentence isn't plural, so it can't be translated with a plural.
Sure, that's what the sentence implies, but Duo expects you to simply translate the sentence, not translate something else that you assume is happening. "Sammeln" means collect, not save, so that's what you should write.
Duo is a machine and only has so many translations in its database, so it's best to be fairly conservative with your translations.
Hmm, it may be a stickler; but, picking up the mail may not be the same as collecting it if you threw it away. but, seriously, I think this is close enough in meaning as far as translating it for someone as long as they understood you.
Using "pick up" regarding mail is an idiom for "collecting it", in English. So, in the proper context I would say it's acceptable. However, whether Duo accepts it on not would be a different question. Duo doesn't always have every possible translation for a sentence in the selection options section of the answer bot; and so, often choices get overlooked, especially if not that many people use that translation.
Since we have repeatedly learned sammeln, meaning "to collect", I wouldn't always count on Duo having a different answer and most likely they would use the word we've been taught. The one exception to this is if there is a "best" translation. Though subjective in choice in English, often when it's German they will have the more commonly used expression, along with how we might translate it using the words we've learned in the course.
I see you're at level 25 with German, and it's around this time that Duo begins to expand upon subtle concepts and differences in interpretation of meaning. Given that most Duo sentences are without context. The options for which translations are acceptable are often left up to us, the forum users, to discuss and determine which is acceptable. However, that again, doesn't mean that Duo will have added that transation as an acdeptable choice.
I hope that made sense! :-) After all, DuoLingo considers the forum one of the best sources of information for questions regarding any lesson you learn! :-) And, the entire point of Duolingo is to learn. :-))
Duo won't mark you wrong just for a missing umlaut, so you must have had another mistake too; what was your entire answer?
But for what it's worth, the correct way to write the word if you can't type umlauts is "zurueckkommst"; in general, you should substitute "ä/ö/ü/ß" with "ae/oe/ue/ss."
Why isn't it "zurück kommst" (with a space in between)? it is conjugated, not in infinitive form.
As you know, separable prefixes come at the end of a clause, and the conjugated verb comes at the end of a subordinate clause.
When the main verb stem gets "sent to the end" by a subordinating conjunction and meets its separable prefix there, the two join together into one word.