In Exodus 4:10, the anonymous authors of the New World Translation replaced אֲדֹנָי֒ (Ădōnāy), meaning “my Lord”, with Jehovah. So they translate like this:
Moses now said to Jehovah: “Pardon me, Jehovah, but I have never been a fluent speaker, neither in the past nor since you have spoken to your servant, for I am slow of speech and slow of tongue.”
But in the Hebrew text we read:
וַיֹּ֨אמֶר מֹשֶׁ֣ה אֶל־יְהוָה֮ בִּ֣י אֲדֹנָי֒ לֹא֩ אִ֨ישׁ דְּבָרִ֜ים אָנֹ֗כִי גַּ֤ם מִתְּמוֹל֙ גַּ֣ם מִשִּׁלְשֹׁ֔ם גַּ֛ם מֵאָ֥ז דַּבֶּרְךָ ִּ֣אָ֥זי כְבֶּרְךָ אֶל־עַבְדֶּ֑ךָ כִּ֧י כְבַד־פֶּ֛א
As we note from the Hebrew text, in Exodus 4:10 there is only once the tetragrammaton יְהוָה֮ (YHWH) and once only אֲדֹנָי֒ (Ădōnāy). But the authors of the New World Translation also replaced אֲדֹנָי֒ (Ădōnāy) with the wrong form Jehovah.
So the correct translation of the Hebrew text is as follows:
Moses said unto YHWH, My Lord, I am not eloquent, neither heretofore, nor since thou hast spoken unto thy servant: but I am slow of speech, and of a slow tongue.
The same alteration was made by the authors of the New World Translation in Exodus 4:13. There too we find אֲדֹנָי֒ (Ădōnāy) in place of the tetragrammaton יְהֹוָה (YHWH), but in their translation they replace it with the wrong form Jehovah.
Since the Hebrew rule says that the tetragrammaton with the vowels of Ădōnāy must be read Ădōnāy, most translators make the tetragrammaton “Lord”. So we have יהוה (YHWH), אֲדֹנָי֒ (Ădōnāy) and יְהֹוָה (YHWH with the vowels of Ădōnāy, which according to the Hebrew rule should be read Ădōnāy).
Cristiani Cattolici Romani 