The Masculine Genitive & Its Stress Shift

Masculine -ος nouns take -ου in the genitive singular and -ων in the genitive plural: άνθρωπος → ανθρώπου → ανθρώπων, δάσκαλος → δασκάλου → δασκάλων. The endings are the easy half. The trap is the stress: a noun stressed on the third syllable from the end cannot stay there once the ending grows, so the tonos moves one step toward the ending - ά-νθρω-πος becomes αν-θρώ-που, δά-σκα-λος becomes δα-σκά-λου, and the same shift shows in the accusative and genitive plural (ανθρώπους, δασκάλων). Nouns stressed nearer the end have nothing to move: αδελφός → αδελφού → αδελφών is already end-stressed, and δρόμος → δρόμου → δρόμων is already inside the last three syllables. The check to run: form the genitive, then count syllables back from the end - the tonos must land on one of the last three.