Present Participle (not in tree, but here's my stab at cracking an explanation)
I was surprised months after finishing the German tree to learn yet another piece of the grammar puzzle that isn't mentioned at all on Duo: the present participle, oder auf Deutsch, Partizip Präsens (oder Partizip I... eins).
In English, we've got both gerund (verbs used as nouns, like "running is outlawed"), and present participle (verbs typically used as adjectives/adverbs, like "...the blowing wind").
Unfortunately for English speakers, this is a totally unimportant distinction for most of us because they both end in -ing, and look absolutely identical. But not so in German!
Partizip Präsens works like this: take the infinitive of the verb (let's say "blasen", for "blowing" above). Add a -d to the end, and voila, it's now blasend! Now pair it with a noun (with those adjectival endings) and you're in business:
Der blasende Wind. --> The blowing wind. Mit dem blasenden Wind. --> With the blowing wind.
The gerund, on the other hand, turns verbs into nouns by capitalizing them and assigning them a gender of neutered. Einfach:
Das Rennen auf dem Gras ist verboten. --> Running on the grass is forbidden.
I haven't seen this discussed much in the forums, and thought I'd take a crack at it. I'd be happy to correct any errors.
To add to that (but correct me if I'm wrong?):
The Partizip I can also be used to form a noun to describe "the thing" that is doing the action.
It is a peculiar case, as it's a noun that must be declined according to adjectival declension rules e.g.
"Der Reisende / "Ein Reisender hat kein Geld" (The traveller / a traveller has no money). "Reisen" --> to travel.
Don't confuse this with the gerund form noted above.
I think the reason why this is not covered in Duolingo is that it's simply not used much in German. What you're written here is absolutely correct, but even though this usage is correct in German, it's not idiomatic. Languages have forms which tend to become common among native speakers, and other forms which are technically correct but so disused that their usage usually marks the speaker as a non-native speaker. Partizip I is one of these: because English speakers often use this kind of form, they may get into the habit of over-using Partizip I in German, which will mark them as a non-native speaker of German, just as English tend to over-use Nebensätze that begin with "dass" because that form is also common in English. Germans would be more likely to say something like "Der Wind, der bläst" rather than "Der blasende Wind". The latter would immediately cause Germans to say "Uh, that person learned German out of a textbook." So while your information is correct, and good to know since it's occasionally seen in some German writing, at least in spoken German this form is very rare, and native English speakers who are used to this convention in English should avoid falling into the habit of using it in German unless you want to sound like a foreigner.