John 1:34

κἀγὼ ἑώρακα, καὶ μεμαρτύρηκα ὅτι οὗτός ἐστιν ὁ υἱὸς τοῦ θεοῦ.

κἀγὼ pronoun "And I/Even I/I myself"

ἑώρακα verb perfect active indicative 1st singular from ὁράω (I see) "I have seen"

καὶ conjunction "and"

μεμαρτύρηκα verb perfect active indicative 1st singular from μαρτυρέω (I testify/I witness) "I have testified"

ὅτι conjunction "that"

οὗτός demonstrative pronoun "this one"

ἐστιν verb present indicative 3rd singular from εἰμί (I am) "he is"

ὁ definite article nominative masculine singular "the"

υἱὸς noun nominative masculine singular "son"

τοῦ definite article genitive masculine singular "the"

θεοῦ noun genitive masculine singular "of God"

Rough Word-by-word:
And I/Even I/I myself I have seen and I have testified/I have witnessed that this one he is the son the of God

Smooth Translation:
I myself have seen and have testified that this one is the Son of God.

Notes: I love the use of the perfect here. John wants there to be no doubts about who he says that Jesus is.