John 1:8

οὐκ ἦν ἐκεῖνος τὸ φῶς, ἀλλ' ἵνα μαρτυρήσῃ περὶ τοῦ φωτός.

οὐκ negative particle "not"

ἦν verb imperfect indicative 3rd singular from εἰμί "was"

ἐκεῖνος demonstrative pronoun nominative masculine singular "he"

τὸ definite article nominative neuter singular "the" (note:greekbiblestudy.org parses this as nominative, but I would call it accusative here...the form is the same. Perhaps it is just a predicate nominative. I'll do some more checking on this)

(ok, postnote: evidently when you have an "equative" verb like εἰμί the noun is a predicate nominative...something to tuck away in my grammer memory.)

φῶς noun nominative neuter singular "light"

ἀλλ' strong negative conjunction "but"

ἵνα conjunction subjunctive case "that"

μαρτυρήσῃ verb aorist active subjunctive 3rd singular from μαρτυρέω "he might testify"

περὶ prepostion with genitive "concerning"

τοῦ definite article genitive singular neuter "the"

φωτός noun genitive singular neuter "light"

Rough Translation:

not was he the light but that he might testify concerning the light

Smooth Translation:

He was not the light, but that he might testify concerning the light