A relative pronoun agrees with its antecedent in gender and number. Its case is determined by its function in the clause it begins (cf. BBG 14.10).
| Matthew 1:23 | Matthew 1:23 |
| Ἰδοὺ ἡ παρθένος ἐν γαστρὶ ἕξει καὶ τέξεται υἱόν, καὶ καλέσουσιν τὸ ὄνομα αὐτοῦ Ἐμμανουήλ, ὅ ἐστιν μεθερμηνευόμενον Μεθʼ ἡμῶν ὁ θεός. | “Behold, the virgin ⌊will become pregnant⌋ and will give birth to a son, and they will call his name Emmanuel,”which is translated, “God with us.” |
Notice first that ὅ is masculine, singular which agrees with its antecedent Ἐμμανουήλ which is also masculine, singular.
Then notice that ὅ is also the subject of the verb ἐστιν and hence is in the nominative case.