"I will sit and work."
Translation:Bidh mi a' suidhe agus ag obair.
Obair is a verbal noun meaning act of working, ag obair is literally at work, at working. The verb associated with it is oibrich to work. Just obair wouldn’t make any grammatical sense here – it is a noun, and a noun cannot be a predicate of the bi verb,
*bidh mi obair would sound like a very ungrammatical attempt at saying *I will be work.
Bidh mi a’ suidhe agus ag obair means I will be sitting and working – you could translate it that way too and I think it would be accepted, not entirely sure what is the contributors’ policy for accepted future tense translation though – I see the bidh mi a’ dèanamh… construction often translated as simple future in English: I will do… instead of I will be doing…. Anyway, Duolingo doesn’t teach the simple future at the moment, only the bidh … ag … construction.
I will sit and work hypothetically could be also said as Suidhidh (mi) agus oibrichidh mi with future tense of the verbs – but I think people rather generally say ag obair when speaking about working than the verb oibrich directly (there are almost no Google results for the phrase oibrichidh mi).