My grammars tell me that both Homer and Herodotus have the present indicative 2nd singular form of εἰμί in εἶς. In my text, however, the form is found without an accent (e.g. εἰ δὴ ἐξ αὐτοῖο τόσος πάϊς εἰς Ὀδυσῆος, Od. 1.207). Why is this?
Present tense forms of εἰμί — including Homeric 2d sing. εἶς – are normally enclitic, i.e., unaccented, except Attic 2d sing. εἶ. There are some situations where these forms accented, but this isn’t one of them. Consult a grammar for more information.