From H. W. Smyth, Greek Grammar, I §§149-188
GENERAL PRINCIPLES, ANASTROPHE, CHANGE OF ACCENT IN DECLENSION, INFLECTION AND COMPOSITION, PROCLITICS, ENCLITICS
172. If neither of the syllables to be contracted had an accent, the contracted syllable has no accent: φίλεε = φίλει, γένεϊ = γένει, περίπλοος = περίπλους.
173. Crasis. – In crasis, the first word (as less important) loses its accent: τἀ̄γαθά for τὰ ἀγαθά, τἀ̄ν for τὰ ἐν, κἀ̄γώ for καὶ ἐγώ.
a. If the second word is a dissyllabic paroxytone with short ultima, it is uncertain whether, in crasis, the paroxytone remains or changes to properispomenon. In this book τοὔργον, τἄ̄λλα are written for τὸ ἔργον, τὰ ἄλλα; but many scholars write τοὖργον, τἆλλα.
174. Elision.– In elision, oxytone prepositions and conjunctions lose their accent: παρ' (for παρὰ) ἐμοῦ, ἀλλ' (for ἀλλὰ) ἐγώ. In other oxytones the accent is thrown back to the penult: πόλλ' (for πολλὰ) ἔπαθον.
a. Observe that in πόλλ' ἔπαθον the acute is not changed to the grave. A circumflex does not result from the recession of the accent. Thus, φήμ' (not φῆμ') ἐγώ for φημὶ ἐγώ. τινά and ποτέ, after a word which cannot receive their accent, drop their accent: οὕτω ποτ' ἦν.
Reference address : https://www.ellopos.net/elpenor/lessons/greek-accentuation.asp?pg=6