I find memorising the vocabulary of Greek much harder than that of Latin, which for a native English speaker must be the friendliest of any language. But, according to Wilfred E. Major, there is some solace:
One area where Greek is quantifiably easier than other languages is vocabulary, or, more exactly, frequency of core vocabulary. In English, the 100 most common lemmas typically constitute nearly half of a text. Thus words like “the,” “an” and “is” appear many times, while words like “paraclausithyron,” sadly, occur infrequently. Furthermore, 80% of English texts comprise fewer than 2400 words. This total is not unusual; many languages have a core vocabulary of 2000–3000 lemmas which generate 80% of written texts. Latin, for example, is comparable to English in this regard.
Greek is atypical, but in a useful way. 65 or fewer lemmas generate half of Greek texts, so I suggest that students should learn these words early and practice them often. Students are certain to encounter these lemmas constantly, and comfort with them will provide a reasonable basis as they negotiate original texts. Experienced readers of Greek will find no surprises on this “50%” list, but students in Beginning Greek may. Verbs such as γίγνομαι and δίδωμι, nouns such as ἀνήρ, pronouns like οὐδείς, and the host of prepositions and conjunctions are rarely introduced early and reinforced often.
An 80% list for Greek also repays attention. A vocabulary list generated by Perseus yields between 1000 and 1100 words, less than half of the standard 2000–3000 for many languages.
That quotation comes from this pdf
. The 50% and 80% vocabulary lists can be found in this pdf