Regarding John 10:30, ESMEN is the first-person plural indicative active form of EIMI. It means "we are." Therefore, "the Father and I are (ESMEN) one."
Some theologians and exegetes take the position that the Father and Son are one in substance or being. For instance, Charles Ryrie believes John 10:30 puts forth the view that the Father and the Son are one in substance. Rudolf Schnackenburg additionally writes that 10:30 gives us a glimpse of "the metaphysical depths contained in the relationship between Jesus and the Father." Conversely, Baptist exegete Gerald Borchert, while wanting to avoid an "Arian" interpretation of the verse explains: "the word 'one' here is neuter (hEN) and not masculine (hEIS), so the text is not arguing for a oneness of personalities or personae . . . but rather something akin to a oneness of purpose and will" (Borchert 341).
After reviewing the evidence, I opt for Borchert's view (although I draw differing implications from this verse than Borchert does). One Johannine verse that governs my thinking is John 17:20-22. In prayer with his disciples, Jesus prays that his followers may be one AS he and the Father are one. The unity discussed in John 17:20-22 is not an ontological unity, but a unity of action and purpose. I take it that Jesus was discussing the same type of unity in John 10:30 (Cf. 1 Cor. 3:8). The early church father, Novatian, also interpreted John 10:30 in this manner:
"But since they frequently urge upon us the passage where it is said, 'I and the Father are one,' in this also we shall overcome them with equal facility. For if, as the heretics think, Christ were the Father, He ought to have said, 'I and the Father are one.' But when He says I, and afterwards introduces the Father by saying, 'I and the Father,' He severs and distinguishes the peculiarity of His, that is, the Son's person, from the paternal authority, not only in respect of the sound of the name, but moreover in respect of the order of the distribution of power, since He might have said, 'I the Father,' if He had had it in mind that He Himself was the Father. And since He said 'one' thing, let the heretics understand that He did not say 'one' person. For one placed in the neuter, intimates the social concord, not the personal unity. He is said to be one neuter, not one masculine, because the expression is not referred to the number, but it is declared with reference to the association of another. Finally, He adds, and says, "We are," not "I am," so as to show, by the fact of His saying" I and the Father are," that they are two persons. Moreover, that He says one, has reference to the agreement, and to the identity of judgment, and to the loving association itself, as reasonably the Father and Son are one in agreement, in love, and in affection . . . In receiving, then, sanctification from the Father, He is inferior to the Father. Now, consequently, He who is inferior to the Father, is not the Father, but the Son; for had He been the Father, He would have given, and not received, sanctification" (De Trinitate XXVII.16ff)