there should be more explaination, for example (em + o)=no, (em+a)=na, em + uma=numa etc.
You're right. Duo does not explain rules on grammar (especially the Portuguese section). But if you go through the discussions you'll find many interesting tips on grammar, including prepositions+articles ;)
I wrote 'Eles correm...' which I think should have been accepted but wasn't. I reported it. Was I wrong?