What is YHWH? What is the tetragrammaton?

Jehovah. This is a name of relationship with men, especially with Israel, taken by God in time. It is derived from havah, 'to exist,' and may be expanded into 'who is, who was, and is to come.' God thus reveals Himself in time as the ever-existing One: that is, in Himself eternally, He is always the same: cf. Heb. 1:12. ... "God said unto Moses, I AM THAT I AM," Ex. 3:14, where the word is Ehyeh, which is from the same root as Jehovah, the Eternal existing One; He that was, and is, and the coming One.

Unfortunately the name Jehovah is seldom employed in the A.V. It is generally represented by LORD (sometimes GOD) printed in small capitals. There is a contraction of Jehovah into Jah, also translated in the A.V. by LORD, except in Ps. 68:4, where Israel is exhorted to sing unto God, and "extol him by his name JAH." Jah signifies the absolute supremacy of the self-existing One; whereas Jehovah was the name made known to Israel, and on which they could count.

The true pronunciation of Jehovah is declared to be lost: the Jews when reading the O.T. never utter it (from a constrained interpretation of Lev. 24:16), but say, 'the name,' 'the great and terrible name,' etc.

The Jews come to treat Yahveh (Jehovah) as taboo, and to employ it in the Temple services only, but in those of the synagogues Adonay (Lord) alone: ... In common life they spoke of "the Name" (Aramaic: Shema). In the Tosefta "Sanhedrin" (xii. 25) we have, "He that pronounces the Tetragrammaton has no part in the future world."

In Genesis 1 it is God in His supremacy, creating by His word. In Genesis 2 and Genesis 3 it is God placing man, His intelligent and responsible creature, in relation with Himself — whether in his original innocence or afterwards in his fallen condition — hence Jehovah comes in, since this name sets Him forth as self-existing, unvarying, faithful to His covenant.

To Moses He said, "I appeared to Abraham, to Isaac, and to Jacob, by the name of God Almighty, but by my name JEHOVAH was I not known to them" (Ex. 6:3). Notice the exact wording here. He did not say, "They did not know My name Jehovah." Abraham knew the name Jehovah, for in Genesis we find him using it. He did not, however, know God by that name; that is, the real meaning and import of the name Jehovah never dawned upon him, inasmuch as the circumstances which demanded such a revelation had not arisen. But now the moment had come for it to be unfolded, and the Almighty One stood forth, pledged in connection with Israel, as the I AM — the self-existent and therefore unchanging One, always true and faithful to His word.

When God has a people on earth, He is made known as Jehovah, or I AM; and when they are redeemed out of Egypt, He dwells among them, and maintains covenant relationship with them as Jehovah. This goes on as long as He can own them as His people, and when He can no longer say of them, "my people," even then, instead of utterly giving them up, Jesus is born into the world to "save his people from their sins." He is called Jesus, or Jehovah Saviour.

In the first book the prevailing title by far is Jehovah, because the godly Jews are looked at as in Jerusalem, and still in association with the temple; whereas in the second book the title most commonly used is not Jehovah, but God. And why? Because the remnant of Jews are looked at there as having fled from Jerusalem ... and their covenant relationship with Jehovah is little realised by them.

Of old Jehovah had called Himself "I AM." Now the Word has been made flesh, and He too is "I AM," but He fills it out in detail. Here we have, "I AM the resurrection and the life."