Translation:We were going to begin with playing soccer, but he had forgotten the ball at home.
It sounds a bit clunky to me, too. I think "he had forgotten the ball" is fine, or "he had left the ball at home", but "forgotten the ball at home" seems a strange thing to say.
I'm happy to report that "left the ball at home" was accepted, so I know I don't need to remember the strange phrase, and maybe that's how some people do say it.
For me the whole sentence was already written and I only had to press "check" or whatever it is called. Weird?
You've never forgotten anything at home and not realized it until you needed it later? Are you from the UK, too (see above)? The first part of the sentence sounds a little awkward to me, but not the second.
"Hey, kids, what do you want to play first today?" "Let's start with football!" "OK - great - let me get my ball from my bag. (Rustle, rustle.) Where ... oh, no, I took it out last night and forgot it at home! Guess we'll have to start with innebandy."
I am trying my hardest to think of an example where this might work, but I can't!
I would still use 'left it at home' rather than 'forgot it at home' in your example!
I think it must be a English English versus American English divergence. Anyhow, it's good for me to learn the varying uses around the world!
As estersandstrom suggests below, I think this sentence is about playing several sports or games, of which football is (or was going to be) the first.
The English sentence has "with" in it, the same as the "med" in the Swedish. It's something more specific than just starting to play. If the sentence here doesn't quite make sense, here's one with more context: "We took lots of sports equipment to the park, since we planned to stay all day. I was most looking forward to the badminton, but everyone else wanted to begin with playing football."
So yes, you can say "börja spela" but it doesn't mean exactly the same thing, and you don't use "att" in that case. See this helpful comment: https://www.duolingo.com/comment/7075383 for more information on when "att" is used.
Yes, "We were going to start playing football..." ought to be accepted. Just remember that "Vi skulle starta spela fotboll" sounds a bit odd in Swedish, even though "starta" is the usual Swedish translation of "to start". That might be the reason why they haven't included "We were going to start playing football..." among the accepted answers.
Your version would be "Vi skulle börja spela fotboll, men han hade glömt bollen hemma". I don't think you can use "be about to" in this case, as the Swedish version doesn't have that immediate implication.
"We were going to start by playing football, but..." would be another way of translating the Swedish phrase (not sure if it's accepted on DuoLingo though).
Two different meanings. One is "begin playing football" where the group had only planned football, and they start. The other is "begin by playing football" where the group has several things planned, maybe even a whole day of sports with prizes or something, and they play football as the first of those things. Weird sentence, but potentially valid. Consider something like "We were going to begin with serving drinks, but we couldn't find the glasses, so we went straight on to the sandwiches."