A.T. Robertson Commentary 1 John 5:20

"And we know that the Son of God is come, and hath given us an understanding, that we know him that is true, and we are in him that is true, [even] in his Son Jesus Christ. This is the true God, and eternal life." — 1 John 5:20 (ASV)

Is come (ηκε). Present active indicative, but the root has a perfect sense, "has come." See εξηλθον κα ηκω in Joh 8:42.

An understanding (διανοιαν). Here alone in John's writings, but in Paul (Ephesians 4:18) and Peter (1 Peter 1:13). John does not use γνωσις (knowledge) and νους (mind) only in Re 13:18; 17:9.

That we know (ινα γινωσκομεν). Result clause with ινα and the present active indicative, as is common with ινα and the future indicative (John 7:3). It is possible that here ο was pronounced ω as a subjunctive, but many old MSS. have ινα γινωσκουσιν (plainly indicative) in Joh 17:3, and in many other places in the N.T. the present indicative with ινα occurs as a variant reading as in Joh 5:20.

Him that is true (τον αληθινον). That is, God. Cf. 1:8.

In him that is true (εν τω αληθινω). In God in contrast with the world "in the evil one" (verse 19). See Joh 17:3.

Even in his Son Jesus Christ (εν τω υιω αυτου Ιησου Χριστω). The αυτου refers clearly to εν τω αληθινω (God). Hence this clause is not in apposition with the preceding, but an explanation as to how we are "in the True One" by being "in his Son Jesus Christ."

This (ουτος). Grammatically ουτος may refer to Jesus Christ or to "the True One." It is a bit tautological to refer it to God, but that is probably correct, God in Christ, at any rate. God is eternal life (John 5:26) and he gives it to us through Christ.