It is true that the Jews are loath to pronounce God's covenant name, Yahweh. Even in writing, it is not uncommon to see the word God shortened to G-d.
The "tetragrammaton" (from the Greek for four-lettered word) refers to the four consonants which make up the word for God in the Hebrew Old Testament. They are Y-H-W-H. Truth is, no one can say with complete certainty how this was pronounced, since the text lacks vowels. Without the vocalization, we can only guess. Scholars today posit that the original pronunciation was Yahweh.
The tetragrammaton derives from the Hebrew verb to be, and refers to God's existence, which is eternal and self-sustained. There are special echoes of the Name in the Gospel of John, where over and over Jesus says, "I am" (the bread of life, the resurrection and the life, the gate, etc). In John 8:58, the Lord says, "Before Abraham was, I am." This was considered blasphemous, because it was a direct claim to divinity.
At any rate, "Jehovah" is a total mispronunciation of the Hebrew, so the Jehovah's Witnesses are being quite inconsistent when they insist on a particular pronunciation for God's name--and then fail to get it right!
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