Exegesis:
The commandment quoted is from Lev. 19.18, and follows the Septuagint literally.
plēsion (12.33) is an adverb used as a noun, meaning ‘neighbor.’ In this context it does not indicate simply the person who lives next door, but represents the Hebrew reʿa ‘fellow citizen,’ or, in a more general way, ‘the other man,’ ‘fellow-man.’
hōs seauton ‘as yourself’: in the same way a man loves himself he is to love his fellow-man.
Translation:
The second may require the addition of the noun ‘commandment,’ sometimes with a shift in order of subject and predicate elements, e.g. ‘this is the second commandment.’
The form of this command is in the singular, including not only the subject you but the object neighbor. In many languages, however, this use of the singular would be understood to mean that Jesus was telling the particular man that he should love his one neighbor. In order for this to be generic, many languages require plurals, e.g. ‘you (plural) should love your neighbors.’
The English form employs the future auxiliary shall but the meaning is obligatory, which in many languages must be rendered as ‘must,’ ‘ought to,’ or ‘should.’
For love see 10.21.
Neighbor is usually quite easily translated, but there are sometimes certain idiomatic forms which are employed, e.g. ‘a person outside of your building’ (Barrow Eskimo), ‘your back and side,’ implying position of the dwellings (Tzeltal), ‘younger-brother-older-brother,’ a compound which means all one’s neighbors in a community (Kekchi).
As yourself may require the repetition of the verb in order to show the parallelism, e.g. ‘feel hurt for your neighbors as you feel hurt for yourselves’ (Tzeltal).
For constructions involving comparison see 1.7. In this instance one can sometimes translate as ‘no other commandment surpasses these two commandments’ or ‘these two commandments are really big; no other commandment is big’ (where a paratactic positive-negative statement is the normal construction).
Quoted with permission from Bratcher, Robert G. and Nida, Eugene A. A Handbook on the Gospel of Mark. (UBS Handbook Series). New York: UBS, 1961. For this and other handbooks for translators see here .