So, the basic rule starts with the “νόμος της τρισυλλαβίας” (nómos tis trisyllavías), the “three-syllable rule” according to which the stress of a multi-syllable word must occur on one of the last three syllables:
σημαντικός (simandikós), "important" [Stress on last syllable]
δασκάλα (daskála), "teacher" (fem.) [Stress on second-last syllable]
μάθημα (máthima), "lesson" [Stress on third-last syllable]
A. When a word acquires additional syllables, e.g., to form the plural, then the stress mark will shift to ensure that the rule is preserved:
μάθημα (máthima) in singular becomes μαθήματα (mathímata) in plural.
B. When a word attaches to another as to syntactically (and phonologically) form one word, then the
first word may acquire a second stress or the second word that is normally unstressed, is stressed.
This applies within a phrase containing one or more weak pronouns, such as, me, se, ton/tin; mou, sou, tou/tis, etc.
1. Noun/adjective/adverb/verb followed by possessive pronoun:
O δάσκαλος + μας => O δάσκαλóς μας (o dáskalós mas), “our teacher (masc.)”
2. Imperative verb followed by pronoun/s:
Κάλεσε + την => Κάλεσέ την (kálesé tin), “invite her!” [Original stress on third-last syllable]
Πάρε + του + το => Πάρε τού το (páre toú to), “take it from him!” [Original stress on second-last]
Γράψε + μου + το => Γράψε μού το (grápse moú to), “write it to me!” [Orig. stress on second-last]
3. Adverb/gerund used adverbially followed by pronoun/s:
Απέναντι + σας => Απέναντί σας (apénandí sas), "opposite you"
Δίνοντας + σου + τα => Δίνοντάς σου τα (dínondás sou ta), “giving them to you”
This applies to words stressed on the second-last or third-last syllable.
When the word is stressed on the last syllable, the stress does not change E.g.:
Aγαπητός/αγαπητή + μου = Aγαπητός/αγαπητή μου (agapitós/agapití mou), “My dear/favourite (masc./fem.)”