A.T. Robertson Commentary Romans 13:9

"For this, Thou shalt not commit adultery, Thou shalt not kill, Thou shalt not steal, Thou shalt not covet, and if there be any other commandment, it is summed up in this word, namely, Thou shalt love thy neighbor as thyself." — Romans 13:9 (ASV)

For this (το γαρ). For the article (το) pointing to a sentence see 8:26, here to the quotation. The order of the commandments here is like that in Lu 18:20; James 2:11 and in B for De 5, but different from that of the Hebrew in Ex 20; Deuteronomy 5. The use of ου with the volitive future in prohibitions in place of μη and the imperative or subjunctive is a regular Greek idiom.

And if there be any other (κα ε τις ετερα). Paul does not attempt to give them all.

It is summed up (ανακεφαλαιουτα). Present passive indicative of ανακεφαλαιοω, late literary word or "rhetorical term" (ανα, κεφαλαιον, head or chief as in Heb 8:1). Not in the papyri, but κεφαλαιον, quite common for sum or summary. In N.T. only here and Eph 1:10.

Namely (εν τω). See το γαρ at the beginning of the verse, though omitted by B F. The quotation is from Le 19:18. Quoted in Mt 5:43; 22:39; Mr 12:31; Luke 10:27; Galatians 5:14; James 2:8 it is called βασιλικος νομος (royal law).

Thy neighbour (τον πλησιον σου). Πλησιον is an adverb and with the article it means "the one near thee." See on Mt 5:43.