Proposal - idiomatic sentences and expressions
If that kind of sentence is shown for the first time, always show it through Listen and type exercise and perhaps include additional notice "Idiomatic sentence" :)
This may be a problem if idiomatic sentence contains a word never taught before. If it does, don't show that idiomatic expression until that word is taught in non-idiomatic sentence.
This way people will have opportunity to learn idiomatic expressions without losing a heart (without getting frustrated) when their grammatically correct translation wasn't accepted since they stumbled upon an idiomatic sentence (doesn't happen very often). And also this way they won't miss the idiomatic translation (shown in another correct translation) when their grammatically correct translation is accepted.
I think another approach could be to have proverbs ("idiomatic sentences") displayed each time we finish a module which is relevant to that module.
Then have a final module at the end of the tree for proverbs. One final challenge, unique from all the rest, standing in the way of completing the tree.