"Let's go to the library with the dog."
Translation:개하고 도서관에 다녀요.
No it should be 개하고 도서관에 가자. 개하고 도서관에 다녀요 just means "(I) go to the library with the dog"
가자 is in 해라체. The ending -어요 can also be used in the propositive mood in 해요체.
I think the main objection is to the verb, not to the ending, which could also be 갑시다. There is nothing in the English sentence to suggest repeated trips, a routine, or even a return.
I agree with you roberto727. The Korean answer here implies that it's occuring frequently, whereas the English is inferring a one-off journey.
the previous comment by okottacat is correct it should be kae hago do so gwan ae so kap shi da or kaja. Even the Koreans don't use that phrase. I might be a dead phrase from the Korean war and before. Even In the early 1990's they didn't use that phrase commonly.
I wonder if they have Koreans and English speakers working together on this or just Korean's I taught English for 8 years in Seoul and the English teachers we worked with the Korean teachers to iron out the kinks in exactly what some phrase were and the actual sentence that was used in most often in the same situation in the other language. Anyway this is beta the time to use us learners small adjustments that will help. If I was adding content I would be reading the discussion boards all day long...
Is 다녀 some form of the verb 다니오다? Meaning "going there and back" or something like this?
다니다 is almost of like "attend" but you will find it in a lot of compound verbs like 단녀오다 and 단녀가다 to visit and then come/go, and others like
찾아다니다 to go searching around
뛰어다니다 to jump or run around
떠돌아다니다 to wander around etc