Most words in ancient or koine Greek – as typeset in modern days, by convention – have an accent. The exceptions include:
(1) enclitics like the indefinite pronoun (τις, τι), some forms of the personal pronoun (e.g., μου, σου) and the disyllabic forms of the present indicative of εἰμί*.
(2) proclitics like εἰ, ‘if’, and some forms of the article (e.g., ὁ, ἡ, οἱ, and so on).
What is the difference between an enclitic and a proclitic? Enclitics are considered to be closely associated with their preceding word in a phrase – they lean back – when it comes to accentuation. Proclitics are considered to be associated with the following word – they lean forward.
The rules for accent take both these types into account, and complications ensue. For our purposes in looking at Matthew 18: 28, we may note the following:
The phrase Ἀπόδος εἴ τι ὀφείλεις (‘Pay if any you owe’, or ‘Pay what you owe’) contains a proclitic (εἰ) followed by an enclitic indefinite pronoun (τι). In this circumstance one can think of εἰ and τι as one word for purposes of accent, and the proclitic εἰ adds the acute.
Here is similar example, from Mark 4: 23:
Εἴ τις ἔχει ὦτα ἀκούειν ἀκουέτω.
If anyone has ears to hear, let them hear.