John 6:29

ἀπεκρίθη Ἰησοῦς καὶ εἶπεν αὐτοῖς· τοῦτό ἐστιν τὸ ἔργον τοῦ θεοῦ, ἵνα πιστεύητε εἰς ὃν ἀπέστειλεν ἐκεῖνος.

Rough Word-by-word:
answered Jesus and he said to them, "This is the work the of God, that you all might believe into whom he sent he."

Smooth Translation:
Jesus answered and said to them, "This is the work of God, that you might believe in whom he, himself sent."

Notes:
It seems the translations I've checked want to take ἐκεῖνος as an accusative and translate "that you might believe on him or that one whom he has sent." It could be that they are taking the masculine from the relative pronoun ὃν and translating it as "him whom," but then I don't know what they are doing with ἐκεῖνος unless they are ignoring it.

I'm taking it as a nominative being used for emphasis.

ἀπεκρίθη verb aorist passive indicative 3rd singular from ἀποκρίνομαι (I answer) "he answered"

Ἰησοῦς proper noun nominative singular masculine "Jesus"

καὶ conjunction "and"

εἶπεν verb aorist active indicative 3rd singular from λέγω (I say) "he said"

αὐτοῖς pronoun dative plural masculine "to them"

τοῦτό demonstrative pronoun nominative singular neuter "this"

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

τὸ definite article nominative singular neuter "the"

ἔργον noun nominative singular neuter "work"

τοῦ definite article genitive singular masculine "the"

θεοῦ proper noun genitive singular masculine "of God"

ἵνα conjunction with subjunctive "that"

πιστεύητε verb present active subjunctive 2nd plural from πιστεύω (I believe) "you all might believe"

εἰς accusative preposition "into, unto"

ὃν relative pronoun accusative singular masculine "whom"

ἀπέστειλεν verb aorist active indicative 3rd singular from ἀποστέλλω (I send) "he sent"

ἐκεῖνος pronoun nominative singular masculine "he" (for emphasis)