Wow! So much nonsense about Koine Greek, which is what the New Testament was written in. Where to start?
We know much about the meanings of the Greek in the New Testament because there are so many secular and manuscripts from the early church available. Although occasionally, in the case of hapax legomena, or words that occur only one time, it is a challenge to know what usage would be the correct one. In any case, whether KJV or modern, translators are faced with the same issues.
This includes everything from how to find a balance with syntax and word order in the Greek, and changing into a form of English that is grammatically correct. I have been told it is better to favour a smooth English translation, so the reader can understand what the message is!
While word order in Greek is substantially different in Greek, it does NOT add words that are not there. A door (thura) is not a rectangular shape in the wall! It is "thura." Or sometimes a gate (pulon). Sorry I am on my phone, and it is too difficult to post the Greek letters!
Greek does not have archaic or obscure word like the KJV does! Scholars have codified every word, every nuance and put them together in lexicons. Greek is much more complex than English with noun, pronoun and adjective cases. But that actually makes translating easier, as you know by the endings and the roots exactly where the words fit grammatically in the sentence, with word order not having to be subject/predicate like English.
KJ English, in the other hand, was the most updated version of English spoken 400 years ago. But the English language changed radically, including the meanings of words, losing second person singular, and new slang and vocabulary not found 400 years ago (Think technology)! Matt 6:6 contains words implying Jesus tells us to go into a closet to pray. I've got a walk-in closet now, but years ago, in various apartments I lived in, the closets weren't wide enough to fit a person praying. In fact, Jesus is saying to go into a hidden room and pray in secret. So don't boast about your praying in front of other! Now people in 16th century England knew the closet was a fairly large room. They had wardrobes for clothes, not closets. So that is an example of a word that has changed meaning. Not so in Greek! The word is "tamaon" and that meaning is frozen in time. Because Koine Greek, once spoken by the whole world, is a dead language, not a constantly evolving language like Englush currently is.
I could go on and on about words that have changed meanings since 400 years ago, but I think you get the idea! Meanwhile, the biggest issue modern translators face is conveying the correct nuances and meanings in modern English, that a static Biblical language has!