"We walked together to the restaurant because we did not have a car."
Translation:Shiúlamar le chéile go dtí an bhialann mar ní raibh carr againn.
The only thing "wrong" (scare quotes intentional) would be that your sentence begins with "We walked to the restaurant together," instead of what the prompt starts with which is "We walked together to the restaurant." Far more often than not, Duo expects answers which maintain the order of modifiers from the original sentence. I don't know of any way in which your provided answer is grammatically incorrect (although I also concede that there are others who know more than I do who might be able to point out some specific error that I'm not seeing). That said, I have found that I get marked "wrong" far less often when I try to maintain the sentence structure of the original sentence as much as I can when going through exercises. While this might be an inelegant answer to your question, it was the one thing that jumped out for me.
Why is the letter "l" in "Shiúlamar" broad? Since the root verb stem is "Siúil," I would expect the synthetic form of the past tense, first-person plural verb to be "Shiúileamar." However, that word is clearly incorrect. Is there any particular reason that the second "i" gets dropped from the root in this synthetic formation?
It's just if you give an answer in the non synthetic form it marks it as incorrect if you do not spell restaurant as "bhnialann". i.e I gave the answer:
"Shiúil muid le chéile go dtí an bhialann mar ní raibh carr againn"
it was expecting: "Shiúil muid le chéile go dtí an bhnialann mar ní raibh carr againn"
I have reported it, but wasn't sure if this was a typo by the course creators or if there was a scenario whereby it would be spelt that way.
It's still not accepting "Shiúil muid le chéile go dtí an bhialann mar ní raibh carr againn"--it stills says it should be bhnialann and not bhialann if you use the analytical form (Shiúil muid). Reported it again.