From H. W. Smyth, Greek Grammar, I §§149-188
GENERAL PRINCIPLES, ANASTROPHE, CHANGE OF ACCENT IN DECLENSION, INFLECTION AND COMPOSITION, PROCLITICS, ENCLITICS
179. Ten monosyllabic words have no accent and are closely connected with the following word. They are called proclitics (from προκλί̄νω lean forward). They are:
The forms of the article beginning with a vowel (ὁ, ἡ, οἱ, αἱ); the prepositions ἐν, εἰς (ἐς), ἐξ (ἐκ); the conjunction εἰ if; ὡς as, that (also a preposition to); the negative adverb οὐ (οὐκ, οὐχ).
180. A proclitic sometimes takes an accent, thus:
a. οὐ at the end of a sentence: φῄς, ἢ οὔ; do you say so or not? πῶς γὰρ οὔ; for why not? Also οὔ no standing alone.
b. ἐξ, ἐν, and εἰς receive an acute in poetry when they follow the word to which they belong and stand at the end of the verse: κακῶν ἔξ out of evils.
c. ὡς as becomes ὥς in poetry when it follows its noun: θεὸς ὥς as a god. ὡς standing for οὕτως is written ὥς even in prose (οὐδ' ὥς not even thus).
d. When the proclitic precedes an enclitic: ἔν τισι.
N. – ὁ used as a relative is written ὅ.
Reference address : https://www.ellopos.net/elpenor/lessons/greek-accentuation.asp?pg=9