I remember being in the MTC (the LDS Missionary Training Center in Provo, Utah), learning Greek, and saying that I found masculine nouns in -ος hardest to remember, because they had the largest number of distinct endings, whereupon my teacher that day said they found those fairly easy to remember since the endings are nearly always the same as the corresponding form of the masculine definite article.
At which point I thought about it and realised they were right. That made those endings easier to remember, since I had already memorised the forms of the article!
On a related note, I remember the lesson where Brother Straddeck introduced the genitive plural. It went something like this: he wrote on the board, in big, letters, "-ων" and said, "That's it! Lesson over!"
That amused me :) Though since I had been reading up on things a little beforehand, I knew that that wasn't the whole story (e.g. you have to know about the -τ- in -μάτων for neuter nouns in -μα, or the final stress in neuter nouns in -ι such as σπιτιών from σπίτι), but it was a good first approximation.
Funny that for that particular form, all nouns use the same ending in some way.
And related to the previous, the book I had been using to learn some grammar was Langenscheidts Praktisches Lehrbuch Neugriechisch (apparently now superseded by this book with a product number one higher).
Though the comments on the book I used at Amazon were fairly negative, I rather liked it -- perhaps because I didn't use it to learn the language (i.e., go through it and make sure I understand and have memorised everything, try to do the exercises, etc.) but went through the entire book at a "reading" (rather than "learning") pace, in order to have seen things before so that they will seem familiar when I actually learn them in class.
So perhaps that's why I didn't mind that the book "seems like a fossil" or "appears not to have been updated since its genesis 40 years ago"; I wasn't relying on the book for vocabulary, but for basics of declension and conjugation and a superficial idea of how the language works, the details to be filled in by class.
Though the book did have one particular effect on me: the book hadn't been updated for the monotonic orthography; there was merely a preface mentioning it, giving a brief overview of it and an example text in monotonic, and finishing with:
Zur Beherrschung des Neugriechischen ist es vorteilhaft, die alte (weit kompliziertere) Setzung der Ton- und Hauchzeichen zu kennen, da bis 1982 das gesamte Schrifttum, von dem viele Werke, z. B. Enzyklopädien, noch lange Gültigkeit haben, mit diesen Zeichen versehen ist.
(For the mastery of the Modern Greek language it is advantageous to know the older, much more complicated, system of accent and breathing marks, since the entire literature until 1982—of which many works, such as encyclopædias, will have currency for a long time—was printed with those marks.)
So reading that book gave me a distinct fondness for (and a knowledge of the basic rules of) the polytonic orthography, and to this day I will nearly always use polytonic when handwriting (but monotonic when typing on a computer since software support, both in fonts and input methods/keyboard layouts, is much better).
Several years later, I heard from another German who had served his mission in Greece that apparently, this book was highly revered among missionaries in Greece; the way I remember his account, he, too, had bought this book, and when other saw it, they said, "Oh! The German yellow book! All missionaries who have had it have become very good speakers of Greek!"
Hearing that amused me :) After all, I liked it, but I didn't know other people had used it, much less that it had acquired such a reputation.
Speaking of polytonic, two books I was glad to acquire during my mission were the full version of Triandafyllidis's Νεοελληνική Γραμματική (in polytonic), and a slightly older version of Langenscheidt's pocket dictionary (which used polytonic in the Greek–German direction and included katharevousa forms in the morphology summary in the appendix). (A companion of mine had it; I traded it in for the newer version of the same dictionary which I had used throughout my mission until that point.)
What can I say; I'm weird.