▼3:9In Latin, when a sentence has a subject, and the next part of that same sentence has a subject, expressed in a pronoun, the second subject is generally the same as the first. So in ‘non potest peccare’ (he is not able to sin), the subject is the same as of the previous verb ‘manet,’ namely ‘semen ipsus’ (the offspring of God). Therefore, we who have been born of God are able to avoid sin because we have he who absolutely cannot sin remaining in us. Those who commit actual mortal sins lose that presence of Christ and of His Spirit within them which is called sanctifying grace or the state of grace.(Conte)

▼3:9 Commits not sin: That is, as long as he keeps in himself this seed of grace, and this divine generation, by which he is born of God. But then he may fall from this happy state, by the abuse of his free will, as appears from Rom. 11.20-22; Cor. 9.27; and 10.12; Phil. 2.12; Apoc. 3.11.(Challoner)