To Tasos Anesiadis’ answer (Tasos Anesiadis’ answer to What are the most difficult things to learn in the Modern Greek language?), I’ll add for Modern Greek:
- The chaos introduced by the clash of spelling pronunciations from Ancient Greek (via Puristic), and vernacular pronunciations. [nd] vs [nð]; clusters like [fθr]; [i] vs [j] before a vowel (άδεια, two syllables as “empty”, three syllables as “day off”).
- The subtler points of when to use the subjunctive (e.g. θέλω αυτοκίνητο που [να] κοστίζει 100 χιλ ευρώ I want a car which costs €100k vs I want a car which should cost €100k—the former implies you have a specific car in mind, the latter is a generic reference).