Actually, I did not say that ESV translated monogenes incorrectly. What I did say is that the ESV translators focused and displayed in English the first part of the compound word mono-genes. The translators may have given the popular trinitarian definition found in trinitarian lexicons, but lexicons do not always show lexical particulars. The ESV translated monogenes as "the only God."
That may be acceptable to Trinitarians, but since the Trinity doctrine was developed after John's writings, one does well to question controversial renderings. The ESV translated like if the original reading was: "ho monos theós." Is this what we find at John 1:18? Please take a look at how monos is applied to God in the next few verses.
"O [Jehovah] our God...you alone [monos] are God" - Is. 37:20, Septuagint.
"Father,....to know thee who alone [monos] art truly God" - John 17:1, 3, NEB.
"from the one who alone [monos] is God" - John 5:44, TEV, cf. NEB, AB.
"he alone [monos] is God" - 1 Tim. 1:17, LB
"to the one only [monos] God" - Jude 1:25, AB, cf. LB.
Now, how should mono-genes be translated? Some Trinitarians avoid the -genes part of the compound. Some may think the -genes part is not necessary to translate the compound word. There may be a doctrinal objection to show a rendering for that part.
Take a look at how the ESV translated a compound word in Matthew 18:9, monó phthalmos = one eye. (mónos & opthalmós). This is in contrast to dúo o phthalm ó s = "two eyes." Yes, this rendering affects no religious doctrine.
Below you will find a list of compound words and their translations. Notice how the second part of the word is not ignored.
m onohḗmeros = one day; a day ( Wisdom of Solomon 5.14, LXX)
monókerōs = one horn ed ; “unicorn” (Job 39.9, ABP, LXX)
monózōnos = one girdle; “a band of troops” (NETS); “ lightly armed ” (2 Samuel 22.30, ABP, LXX.)
Modern Greek :
monokúttaros = one-celled
monódrama = single play, one [person] drama
monóprakto = one-act play
monódromos = one-[way] street
monoet ḗ s = one year
monographía = monograph
monóklino = single-[bed] room
monokommatikós = one party
monópeto = single breasted
monokómmatos = one piece
monóphylos = one-leaf
monochromía = monochrome
monócheiras = one-handed
monóchordos = one-stringed
monosántalos = [wearing] one-sandal
monóstichos = [of] one verse
monótrochos = one-wheeled
It is standard practice to consider the full meaning of Greek compound words.
Many scholars do define monogenes huiós/theós as the "only-begotten Son/God." This way, they are addressing the fact they are in presence of a compound term. Do you not think this affects accuracy?