From here.
A look at the Greek language from the POV of how hard it is for an English speaker to learn it. The truth is that Greek is a very hard language to learn, and Ancient Greek is probably one of the most Godawful languages on Earth to learn.
Hellenic
Greek is a difficult language to learn, and it’s rated the second hardest language to learn by language professors. It’s easy to learn to speak simply, but it’s quite hard to get it down like a native. It’s the rare second language learner who attains native competence. Like English, the spelling doesn’t seem to make sense, and you have to memorize many words. Further, there is the unusual alphabet. However, the orthography is quite rational, about as good as that of Spanish. Whether or not Greek is an irregular language is controversial. It has that reputation, but some say it is not as irregular as it seems.
Greek has four cases: nominative, accusative, genitive and vocative (used when addressing someone). There are three genders: masculine, feminine and neuter. Nouns have several different declension patterns determined by the ending on the noun. Verb conjugations are about as complicated as in Romance. Greek does retain the odd aorist tense. Greek syntax is quite complicated.
Greek gets a 5 rating, extremely difficult to learn.
Classic or Ancient Greek is worse, with a distinction between aspirated and unaspirated consonants, a pitch accent system and a truly convoluted, insanely irregular system of noun and verb inflection. It has a dual number in addition singular and plural and it has a very difficult optative case. Irregular verbs have one of six different stem types. The grammar is one of the most complex of all languages and the phonology and morphology are truly convoluted.
Classic Greek gets a 5 rating, hardest of all.
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The study of classical Greek requires some attention, you cannot learn it without at least a little speck of sacred fire, but it is NOT difficult, quite the contrary, it had and still has the reputation to be picked up quite fast by those who had to use it as a second language in the Antiquity, even though it comprises countless but not counter-intuitive irregularities the Greek themselves learned to master throughout their whole lives (especially since they varied greatly from one dialect to another) but the approximate knowledge of which never bars one from commanding the whole thing at an early time in your study. The declension system is simpler than that of most other like inflected languages, and above all more mathematical (genitive for the place of origin, dative for the place of station, accusative for the place of direction, nominative for the being in motion, few languages offer such a unified view enabling you to know the correct use of each particle at once, contrary to German where accusative is also supposed to indicate direction but not with many particles like zu). It has a magnificently clear sentence structure that could command with equal ease very casual expression and complex philosophical thought, much simpler and above all much more delightful than that of German. Compared to Latin it is most readable, especially since the whole pronunciation and intonation is indicated with a luxury of detail lacking for Latin and Latin languages (despite their being even more “cantabile”) and even more so for English. The proof the language is clear and not so difficult is that the scientists who need their brains for so many other more urgent matters very seldom resist the fascination for it once they have been put into it by their obligatory curriculum or by their own spare time curiosity, to the point they love to coin their new words from that language much beyond what is necessary. Most of the irregularities and subtle points come from the verb system, which looks rather strange compared to many others (especially as regards the aorist tense which can be best described as a non-present), but it is no more irregular that the French language’s where an even greater number of usual verbs need the memorization of five to six unpredictable parts to be mastered, and in Greek that difficulty is compensated by the multitudinous word derivations each irregular part can give way to : once you have mastered the twelve most irregular verbs, which are monstrous indeed, you master nearly half of the vocabulary.