Modal verbs - too late and too little
I've followed two modern Greek courses in real life and the use of modal verbs came up very early on. Constructions using μπορω να and πρεπει να are VERY common in Greek. I'm now nearly at the end of the Duolingo tree and they've only just come up. That means there are next to no exercise where you can practice them. The Greek subjunctive e.g. θα μπουρουσα να isn't dealt with in a separate lesson but is used in a couple of sentences in the same lesson as the modal verbs.
I realise that the Greek course is a Beta version which is why I'm leaving this suggestion for a change - please, please, please introduce both of these language skills much earlier and use them much, much more in the other lessons. These are things that you really need to practice a lot to be able to speak or understand modern Greek.
I agree. Modal verbs should be before teaching verbs, the first verbs you have to learn, after present tense and they are not difficult at all. Specially if the exercises are selected so as not involve difficult verbs, and tenses
About Subjunctive there are some phrases but not only in the modal verbs section but it seems that that it is omitted. The only explanation I can imagine is that this tree has much influenced by the reverse tree. Is it useful? Definitely. Because the verbs have declension and tenses in Subjunctive.