In short

Greek allows the accent to sit on one of the last three syllables of a word — never further back. That single restriction, the νόμος της τρισυλλαβίας (“law of the three syllables”), explains something that looks like an irregularity to a beginner: το μάθημα (“the lesson”) turns into του μαθήματος in the genitive. The word did not change its stem, its gender or its class. It simply grew a syllable at the end, and the accent had to step forward to stay inside the last three.

So you do not memorise “μάθημα has a weird genitive”. You memorise one law and apply it everywhere: nouns, adjectives, verbs, even the little words that hang off the end of a word. Whenever an ending adds a syllable to a word already stressed on the third-from-last syllable, the accent moves one syllable to the right. A second, inherited rule moves it rightwards after a few endings that do not add a syllable — ο άνθρωπος → του ανθρώπου — and the next section keeps the two apart.

The rule

Count syllables from the end of the word. Greek has three positions and a name for each:

There is no fourth position. A form stressed on the fourth-from-last syllable does not exist in Modern Greek.

Now the consequence. Endings in Greek are not all the same length: the neuters in -μα, for instance, build their oblique forms on a stem one syllable longer than the nominative:

• το μά-θη-μα: the accent is on μά, three syllables from the end. Legal.
• add the genitive ending -τος: μα-θή-μα-τος would be *μά-θη-μα-τος with the old accent — four syllables from the end. Illegal. So the accent moves right: του μαθήματος.

That is the whole mechanism. Two things follow from it:

1. Not every form shifts. The accent moves only when it has to. If the new ending does not add a syllable, the law is not broken and nothing happens: διάβαζα → διάβαζαν (still three syllables from the end: δι-ά-βα-ζαν), ο άνθρωπος → οι άνθρωποι (άν-θρω-ποι).

2. Only προπαροξύτονη words are at risk. A word already stressed on the second-from-last or the last syllable has room to spare: ο δρόμος → του δρόμου, των δρόμων — the accent stays put, because δρό-μου is still within the last three. The shift is a problem of words stressed far back.

There is a second, separate force that also pulls the accent rightwards, and it works even when the syllable count does not change. In the masculines in -ος and the neuters in -ο, the endings -ου, -ων and -ους move the accent of a προπαροξύτονη word onto the second-from-last syllable: ο άνθρωπος → του ανθρώπου, των ανθρώπων, τους ανθρώπους — while οι άνθρωποι and τον άνθρωπο keep it. Count the syllables and ανθρώπου is no longer than άνθρωπος; the movement is inherited from Ancient Greek, where those endings held a long vowel that weighed as much as an extra syllable. Learn it as a property of three endings — -ου, -ων, -ους — but of the individual word too: it is the learned half of the vocabulary that moves, while homely words of exactly the same shape keep their accent (ο αντίλαλος → του αντίλαλου, το σύννεφο → του σύννεφου). Store every such noun with its genitive; lessons 28, 30 and 31 come back to this.

A third force attracts the accent onto the ending itself in the genitive plural. η θάλασσα (stressed three back) gives των θαλασσών, not *των θαλάσσων. This holds for most feminine nouns in -α/-η and most masculines in -ης/-ας — but not for all of them (η εικόνα → των εικόνων) — and you will meet it again with each declension.

Finally, the three-syllable law itself is why a Greek word can carry two accents. When a weak pronoun (μου, σου, του, της, μας, σας, τους) attaches to a προπαροξύτονη word, the pronoun behaves like an extra syllable of that word — the group would be stressed four syllables from the end. Greek does not move the original accent here; it adds a second accent on the last syllable of the host word: ο δάσκαλός μου, το μάθημά μας, το αυτοκίνητό του. This δεύτερος τόνος is obligatory, not optional.

Forms & patterns

ο άνθρωπος — the accent shifts in both genitives

Note the accusative plural: τους ανθρώπους takes one of the three accent-moving endings and shifts, while the accusative singular τον άνθρωπο does not. Neither form is longer than άνθρωπος — here it is the ending, not the syllable count, that decides.

το μάθημα — a neuter in -μα, same law

Here the genitive singular and every plural form grow a syllable, so all of them shift; the genitive plural μαθημάτων then moves one step further still, because -ων is one of the accent-moving endings.

ο δάσκαλος and το αυτοκίνητο — the same movement

A word that never moves: ο δρόμος

The genitive-plural pull (a class rule, not a syllable count)

Verbs: the past tenses show it clearly

The imperfect of διαβάζω is stressed on the third-from-last syllable in the singular. In the plural the endings are longer, so the accent moves right:

Notice that the 3rd person plural διάβαζαν / έγραφαν does not shift: it has the same number of syllables as the singular, so no law is broken. (With έγραφα the vowel έ- itself disappears once the accent leaves it — that is the augment, and it comes later in the path.)

The second accent with a weak pronoun

Examples

Common mistakes

• του άνθρωπου -> του ανθρώπου — -ου is one of the endings that pull the accent of this class onto the second-from-last syllable.
• των μαθήματων -> των μαθημάτων — the longer plural stem μαθηματ- has already moved the accent once (μαθήματα), and the genitive-plural ending -ων moves it one more step right.
• των θάλασσων -> των θαλασσών — feminines in -α of this class pull the accent onto the genitive-plural ending; this is a class rule, not just syllable-counting.
• των πολίτων -> των πολιτών — same class behaviour for masculines in -ης.
• οι ανθρώποι -> οι άνθρωποι — -οι adds no syllable and is not one of the accent-moving endings, so nothing moves; do not “fix” a form that was never wrong.
• του δρομού -> του δρόμου — a παροξύτονη word has room; the accent shifts only when the law or the ending forces it.
• διάβαζαμε -> διαβάζαμε — the plural ending -αμε adds a syllable, so the accent must move forward in the imperfect.
• ο δάσκαλος μου -> ο δάσκαλός μου — a προπαροξύτονη host takes an obligatory second accent before a weak pronoun.
• ο φίλός μου -> ο φίλος μου — φίλος is παροξύτονη, so no second accent is added; only third-from-last hosts take one.

Tips

• Always count syllables backwards from the end of the word, not forwards from the start. Greek stress is described from the right edge.
• Before writing a genitive, ask two questions: does the new ending add a syllable, and is it one of -ου, -ων, -ους? If either answer is yes and the word was προπαροξύτονη, move the accent one step right. Otherwise leave it alone.
• Learn the three labels οξύτονη, παροξύτονη, προπαροξύτονη early — grammars, dictionaries and later lessons in this path use them as shorthand for exactly this behaviour.
• Neuters in -μα (μάθημα, όνομα, πρόβλημα, σύστημα) are the most productive shifting class in everyday Greek: their genitive singular -ματος and genitive plural -μάτων force the move every time.
• The genitive plural is where most learners slip, because two different pressures can apply: the accent-moving ending -ων (ανθρώπων, μαθημάτων) and the class-based pull onto the ending itself (θαλασσών, πολιτών). Treat them as separate facts.
• Say the word out loud. Native pronunciation almost never violates the law, so if a form feels impossible to pronounce with the accent where you wrote it, the accent is probably wrong.
• The second accent is written because it is heard: ο δάσκαλός μου really does have two beats. Reading such phrases aloud makes the rule stick faster than memorising it.