Greek phonetic rules: vowels become psilous (smooth) before 's' and 'ψ'; 'a' before 'ψ' and other consonants; dasynesis (aspiration) occurs in forms like ἅψεα and with certain verbs (ἱημι, ἱάλλω); many examples and exceptions provided.
ἀπὸ συμφώνου, ψιλοῦται, ἀντίος, ἐννέα, ἔγχος, τοῦ ἁνδάνω ὑπεξαιρουμένου. * Τὰ φωνήεντα ψιλοῦσθαι θέλει ἐπιφερομένου τοῦ 'σ' ἔχοντος ἓν τῶν δασέων, ὀσφῦς, ὀσφραίνεσθαι, ἀσφόδελος, ἀσφάραγος, ἀσχάλλω, ὀσχεός, ἴσχω, ἰσχύς, ἀσθενής, ἐσθλός, ἄσθμα, ἔσθω, ἔσθος, ἐσθής. Τὸ 'α' 'ι' 'ο' ἐπαγομένου τοῦ 'ψ' ψιλοῦται, ἄψορρον, ἴψας, ὀψέ. τὰ δὲ παρὰ τὸ ἅψω δασύνεται, ἅψεα, ἁψίς. Τὸ 'α' 'ι' 'η' 'ο' πρὸ φωνήεντος ὄντα κατὰ διάστασιν ψιλοῦται, ἀάπτους, ἀήσυλα, ἀΐσσω, ἰάπτω, ἰατρός, ἠΐθεος, Ἠετίων, ἠέλιος, ὀΐω, ὀϊστός. σεσημείωται τὸ ἵημι καὶ ὅσα γέγονε παρ´ αὐτό, ἱερεύς, ἱέραξ, ἱόμωρος. καὶ τὸ ἱάλλω Ἀττικοὶ δασύνουσι. Τὸ στερητικὸν 'α' ψιλοῦται, ἄφιλος, ἄοικος, ἀλίαστος πλὴν τοῦ ἅλυσις καὶ Ἅιδης. * Τὸ 'α' πρὸ τοῦ 'γ' ψιλοῦται, οἷον ἀγανός. * Τὸ 'α' πρὸ τοῦ 'δ' ὂν καὶ μὴ ἔχον στερητικὴν ἔννοιαν ἢ κρᾶσίν τινα δασύνεσθαι θέλει, στερητικὴν μὲν οὕτως, ἀδαής, ἀδρανής, ἀδευκής, ἀδολέσχης, κατὰ κρᾶσιν Ἅιδης. μὴ οὕτως δὲ ἔχον «ἅδε Ἕκτορι μῦθος ἀπήμων» (Il. Μ 80), ἅδος δέ μιν ἵκετο θυμὸν (Il. Λ 88), ἅδην, ἁδινός, ἁδρός, ἁδήν ἡ νόσος. διὸ καὶ τὸ Ἅδωνις κρεῖττόν ἐστι δασύνειν, ἵνα καὶ παρὰ τὸ ἁδεῖν τῇ δαίμονι ἐτυμολογηθῇ. μάχεται τὸ ἀδράφαξυς δεκτικὸν ἔχον τὸ 'α' καὶ δασυνόμενον καὶ τὸ Ἁδρίας ψιλούμενον.
From a consonant, it is written with smooth breathing: ἀντίος, ἐννέα, ἔγχος, with ἁνδάνω excepted.
The vowels are to be written with smooth breathing when there follows σ having one of the aspirates: ὀσφῦς, ὀσφραίνεσθαι, ἀσφόδελος, ἀσφάραγος, ἀσχάλλω, ὀσχεός, ἴσχω, ἰσχύς, ἀσθενής, ἐσθλός, ἄσθμα, ἔσθω, ἔσθος, ἐσθής.
α, ι, ο, when ψ is added, are written with smooth breathing: ἄψορρον, ἴψας, ὀψέ; but the forms from ἅψω are written with rough breathing: ἅψεα, ἁψίς.
α, ι, η, ο, when before a vowel in hiatus, are written with smooth breathing: ἀάπτους, ἀήσυλα, ἀΐσσω, ἰάπτω, ἰατρός, ἠΐθεος, Ἠετίων, ἠέλιος, ὀΐω, ὀϊστός. ἵημι and whatever has come from it are marked as exceptions: ἱερεύς, ἱέραξ, ἱόμωρος; and the Attics write ἱάλλω with rough breathing.
The privative α is written with smooth breathing: ἄφιλος, ἄοικος, ἀλίαστος, except ἅλυσις and Ἅιδης.
α before γ is written with smooth breathing, e.g. ἀγανός.
α before δ, when it does not have a privative sense or some crasis, is to be written with rough breathing; with privative sense thus: ἀδαής, ἀδρανής, ἀδευκής, ἀδολέσχης; by crasis: Ἅιδης. But not so when it is as in “ἅδε Ἕκτορι μῦθος ἀπήμων” (Il. Μ 80), “ἅδος δέ μιν ἵκετο θυμὸν” (Il. Λ 88), ἅδην, ἁδινός, ἁδρός, ἁδήν ἡ νόσος. Therefore it is better to write Ἅδωνις with rough breathing, so that it may also be etymologized from ἁδεῖν with reference to the divinity. There is a conflict in ἀδράφαξυς, which has α capable of being both smooth and rough, and in Ἁδρίας, which is written with smooth breathing.