For example, how would you translate ‘I was suffering’ into Greek?
ἔπασχον
It’s the future of πάσχω that I object to. I can hardly keep πείθω separate in my head because of it.
εἰ τῷ Διαβόλῳ πείσομαι, πείσομαι πολλά.
πείσομαι δὲ μᾶλλον τῷ θεῷ ἢ τῷ διαβόλῳ.
τῇ οὖν ἀληθείᾳ πείσῃ ὥστε μόνον πρόσκαιρα κακὰ πείσῃ σύγε.
Timothée can probably explain the future πεισομαι of πασχω better than I can. Actually it’s the present πασχω that requires more explanation.
“Root” verbs are verbs that are formed directly from inherited Indo-European roots.
Root verbs that are conjugated with the thematic conjugation (-ω verbs in the present tense, i.e., those conjugated with the ε/ο “theme” vowels, as opposed to the “athematic” -μι verbs) can form three thematic stems with different vowel “grades” (sometimes this is referred to as “Ablaut” or “apophony”):
e grade, usual in thematic present stems
o grade, usual in thematic perfect (“second perfect”) stems
zero grade, usual in thematic aorist (“strong” or “second aorist”) stems
(Greek verbs that are root verbs in the present tense don’t necessarily have all three grades–they can, for example, have sigmatic (“first”) aorists or kappatic (“first” perfects).)
We see the vowel gradations clearly in λ-ε-ιπω (present, e grade), ελ-Ø-ιπον (aorist, zero grade), λελ-ο-ιπα (perfect, o grade) (where the root originally contained a resonant, y, reflected in Greek as ι, which could function as a semivowel, or, between two consonants, as a vowel).
The three grades for the verb πασχω are:
πενθ- (e grade).
πονθ- (o grade).
παθ- (zero grade). The α of the zero grade represents the regular Greek development of an earlier syllabic resonant – a nasal phoneme that could function as a vowel between two consonants in an earlier stage of the language.
This verb forms its aorist and perfect thematically, and there is nothing irregular about them: ε-παθ-ον (aorist, zero grade, with augment), πε-πονθ-α (perfect, o grade, with reduplication).
The future πεισομαι represents the development of an e-grade present stem, πενθ-, with the addition of a sigmatic future suffix, πενθ+σ-, which was simplified to πενσ-, and then developed to πεισ- with compensatory lengthening. The simplification of the consonant cluster -νθσ- to -νσ- is a regular development in Greek, and the consequent weakening (“lenition”) of the -ν- in -ενσ- with compensatory lengthening of the vowel -ε- to -ει- is a regular development in Attic Greek. The resulting future stem πεισ- is conjugated in the middle voice, like many Greek futures of active-voice presents.
The present tense πασχω is a little more difficult to explain. It was formed by adding the familiar (thematic) suffix -σκ-ε/ο- to the zero grade (not the e grade) of the root, i.e., παθ+σκ-ε/ο. The consonant cluster -θσκ- was simplified, with transfer of the aspiration of -θ- to -κ-, yielding -σχ-, i.e., πασχω. Why zero grade, rather than e grade? Apparently present tenses formed with the suffix -σκ-ε/ο, sometimes have zero grade.
In contrast, πειθω, πεισω, επεισα, is relatively regular, with sigmatic future and aorist and simplifications of -θσ- to -σ-.