In the Middle Ages, the "Y" sound of the Greek "I" came to be written as either "I" or "J" (for the first letter of words, at least), and "Iesvs" became either "Iesvs" or, more ornamentally, "Jesvs." And, finally, the "v" came to be written as "u" and the name came into its final written form (in English) as "Jesus." (In fact, even the first editions of the King James Version still used the initial "I" instead of the equivalent "J" which shows that it was still pronounced "Yay-soos" in the English of 1611:
"In form, J was originally merely a [more ornamental] variation of `I,' arising in the 14th century .... Not until the middle of the 17th century did this usage [the new pronunciation of the new letter `J'] become universal in English books; in the King James Bible of 1611, for example, the words Jesus and judge are invariably Iesus and iudge." - p. 4823, Vol. 13, Universal Standard Encyclopedia (Funk & Wagnalls), 1955.
"In the word `hallelujah' the j retains its early consonantal value of i or y." - p. 571, Vol. 15, The Encyclopedia Americana, 1957.
So even for some years after the KJV began using the new letter "J," the pronunciation of it was still "Y." But eventually (18th century?) we began to have "Jesus" (and other "J" words, including "Jehovah," "Jeremiah," "Jerusalem," "Joshua," etc.) with the modern English pronunciation of those letters: "Jee-suz." Nearly all modern English Bibles have purposely retained the earlier tradition concerning biblical names, and "Jesus" (and "Jeremiah," "Jerusalem," "Joshua," etc.) remains in all modern English Bibles.
Many believe that there is nothing wrong with retaining this tradition even though it is not the original pronunciation of the name of the Messiah (Yehoshua) nor even the original Greek rendering of it (Yaysoos). It is still an honest transliteration of the original proper name of the Messiah, however, and it is common to all speakers of English. (In like manner, although `Cristobal Colon' may be the original pronunciation, many do not object to calling the famous explorer `Christopher Columbus' in modern English.)
In the same way the only proper name of God Himself, YHWH, which is used nearly 7000 times in the original writings of the Old Testament is sometimes transliterated as "Jehovah" in English (ASV, Young's, KJIIV, NWT, Byington, and, in some verses only, in NEB, MLB, KJV, and Living Bible) and, more rarely, as "Yahweh" (JB, NJB, and Rotherham). (Of course it is more often improperly rendered "LORD" in most places in most Bibles.)
So which is the proper pronunciation of God's name - "Jehovah" or "Yahweh"? Well, many Bible scholars in more recent times have preferred "Yahweh" as the probable original Hebrew pronunciation. But there is still more to say for "Jehovah" in addition to the fact that it is the older, more traditional, and better-known form.
"In the Elizabethan alphabet the letters 'u' and 'v' were the same letter as were and 'i' and 'j'." - http://www.elizabethan-era.org.uk/elizabethan-language.htm
So "Iehouah" (Yehowah) could also be written "Jehovah."
We are to know and use Jehovah's name. We also must not misunderstand how extremely important it is to Him (and to us). One of God's Ten Commandments, for example commands:
"You shall not misuse the name of Yahweh your God, for Yahweh will not leave unpunished anyone who misuses his name." - Ex. 20:7, NJB [cf. NRSV, NIV, NEB, REB, GNB, NLV, ETRV].
God certainly didn't say, "Don't ever use my Holy Name"! By direct Bible statements and commands and by the clear, thousand-fold repeated examples of all the prophets of God in the OT we know that God's Holy Name must be known and used by his people. (Mt. 6:9) This surely wouldn't mean for it to be withheld from usage or omitted from His Word the Bible, as unfortunately many copyists have kept to following the tradition of eliminating the distinctive name of God by replacing it with Ky´ri·os and The·os´ ("LORD" and "GOD").
God Himself makes it clear in the Bible how important His name is:
"Jehovah ... This is my name for ever; this is my title in every generation." (Ex. 3:15) - NEB
