"Im Wald steht ein Baum."
This wasn't accepted: "A tree is in the wood". In the UK, "wood" is synonymous with "woods", and also "forest" (albeit a wood is a small forest". However, Duolingo doesn't seem to recognise this.
Well, wood is given as a translation for forest. Are you sure it doens't accept your sentence because of the 'is', rather than 'stands'? 'A tree stands in the wood' is a closer translation.
Well, in my case it didn't accept 'A tree stands in the forest', so it's not about the 'is'. It does however suggest 'A tree is standing in the forest', which I think sounds rather odd in English...