Well, you might start by using a literal translation. Then just look at it in context.
(CLV) Mt 5:17
"You should not infer that I came to demolish the law or the prophets. I came not to demolish, but to fulfill.
He came to fulfill both the law and the prophets. That task is a work in progress. We still are awaiting his second coming. Then will come the Millennial reign. The Levitical Priesthood will resume making sacrifices (according to the law) in the Kingdom to come. You see, the law and the prophets are tied together. I suppose that is why Yahshua tied them together in that sentence. Now the Greek word used for fulfill in this sentence is πληρωσαι G4137. (FILL cause, FILL, fulFILL, fill full, complete)
(CLV) Mt 5:18
For verily, I am saying to you, Till heaven and earth should be passing by, one iota or one serif may by no means be passing by from the law till all should be occurring.
The word that the KJV uses for fulfill in this verse, is the Greek word γενηται G1096 (BECOME, come to be, come into being, occur)
It's completely different word; with a completely different meaning. I'm certain that Yahshua chose two different words for a very good reason. King James and his bunch have subtracted from, and added to, scripture.
All has not occurred.
You are following the same anti-Torah bias that King James and his ilk laid out for you to follow. It's a snare.
Prove all things. Hold fast to that which is good.
Shalom