Ten monosyllabic words have no accent and are closely connected with the following word. They are called
The forms of the article beginning with a vowel (ὁ, ἡ, οἱ, αἱ); the prepositions ἐν, εἰς (ἐς), ἐξ (ἐκ); the conjunction εἰ
A proclitic sometimes takes an accent, thus:
a. οὐ at the end of a sentence: φῄς, ἢ οὔ;
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:
d. When the proclitic precedes an enclitic ( cross183 e): ἔν τισι.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].