I have to learn Ancient Greek. So my questions are: Does it help to finish the Duolingo modern Greek tree first? And can someone recommend learning material (either English or ideally in German)?
It depends on how fast you want to learn Ancient Greek. Whenever someone asks me, I always suggest that they learn Modern Greek before Ancient Greek, because by knowing some basic (or more than basic, with some serious practice) Modern Greek they immediately gain access to a lot of learning material, written in Modern Greek, for Ancient Greek. I've heard that Ancient Greek material is hard to find, even in English.
So yes, I'd say it does help quite a bit. ^.^
You're welcome^.^ I wouldn't say they are too different from each other, but then again, I am a native. I can see that the difference would seem way bigger to any non native learner. Also, I know there are a few things that seem quite difficult to everyone, (both for natives and non natives), such as grammar, syntax and some verbs' conjugation rules. But, with the proper help, some good resources (as I mentioned before, there are so many good ones written in modern Greek. Even the ones used in schools are pretty nice and helpful) and a lot of practice, Ancient Greek won't be too hard to learn.^.^
Yes, of course it helps to firstly get into Modern Greek before taking the courageous, brave step to delve into Ancient Greek. Because, they are too many overlapping words between Modern and Ancient Greek. Probably more than half of the whole Modern Greek vocabulary. And many similar grammar rules. About Ancient Greek resources, check this site http://www.lonweb.org/links/link-ancient%20greek.htm If you dig that, it can keep you occupied for a long time! (at least for this year). And here's almost every English root which stems from Ancient Greek, as a Bonus fun resource for you: https://en.wikipedia.org/wiki/List_of_Greek_and_Latin_roots_in_English/A%E2%80%93G https://en.wikipedia.org/wiki/List_of_Greek_and_Latin_roots_in_English/H%E2%80%93O https://en.wikipedia.org/wiki/List_of_Greek_and_Latin_roots_in_English/P%E2%80%93Z