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The LXX translator made a logical change from the Hebrew: valleys should be "filled" or "filled up" not "lifted" or "lifted up." As with the plural paths, the NWT also ignores the Hebrew and follows the Greek. With regard to Luke 3:4 (Isaiah 40:3) it is reasonable to see the LXX translator(s) made a change of a highway to paths because it follows the plural of crooked ways and rough ways, and alludes to Psalm 119. The NWT's claim Jehovah is justified because John was citing the Old Testament in Hebrew is not supported by the entire passage. This is clear even from the NWT which selectively brings looks to the Hebrew text.

The LXX translator made a logical change from the Hebrew: valleys should be "filled" or "filled up" not "lifted" or "lifted up." As with the plural paths, the NWT also ignores the Hebrew and follows the Greek. With regard to Luke 3:4 (Isaiah 40:3) it is reasonable to see the LXX translator(s) made a change of a highway to paths because it follows the plural of crooked ways and rough ways, and alludes to Psalm 119. The NWT's claim Jehovah is justified because John was citing the Old Testament in Hebrew is not supported by the entire passage. This is clear even from the NWT which mostly follows the Greek, only selectively bringing in Jehovah and road(s).

Replacing Jehovah with Lord in English seems to require adding "the" which is not present in any Greek text.

However, the NWT sees the problem with Jehovah Lord and adds words completely foreign to the actual Hebrew (or Greek), But you are Jehovah the Sovereign Lord... Thus, a clear expression of the Psalmist calling Jehovah, Lord asking for mercy, is distorted into a declaration of Jehovah Witness doctrine: But you are Jehovah, the Sovereign Lord.

Replacing Jehovah with Lord in English seems to require adding "the" which is not present in any Greek text. (See examples in the Addendum why this is not the case.)

However, the NWT sees the problem with Jehovah Lord and adds words completely foreign to the actual Hebrew (or Greek), But you are Jehovah the Sovereign Lord... The Psalmist calling Jehovah "Lord" essentially refuting a basic tenet made by Jehovah's Witnesses, is distorted into a declaration of Jehovah Witness doctrine: But you are Jehovah, the Sovereign Lord.

Addendum
Is this claim made by the Watchtower Society valid?

In Greek, is the definite article missing from before Kyʹri·os (Lord), where it would normally be expected grammatically, thus indicating that a proper name may originally have appeared in the Greek text.

In Mark 13 there are 4 instances of the grammatical sequence article-accusative-genitive.

for my name τὸ ὄνομά μου (13:13)
its light τὸ φέγγος αὐτῆς (13:24)
his house τὴν οἰκίαν αὐτοῦ (13:34)
his work τὸ ἔργον αὐτοῦ (13:34)

In each, the translation into English identifies the accusative noun by prefacing it: my name...its light...his house...his work. Therefore, the phrase τὴν ὁδὸν κυρίου must be intelligible in Greek and and may be translated as the Lord's way.

Finally, translating the Hebrew YHVH as κύριος without the article in extremely common in LXX-Psalms. Here is one showing the NWT's altering of an OT text to make it conform to their doctrine:

But you, O GOD my Lord deal on my behalf for your name’s sake; because your steadfast love is good, deliver me!(Psalm 109:21 ESV)
But you are Jehovah the Sovereign Lord. Deal with me for the sake of your name. Because your loving-kindness is good, deliver me. (Psalm 109:21 NWT)
And you O Lord, Lord, do mercy with me for your name's sake, because your mercy is kind. (Psalm 109:21 LXX)

The ESV handles the phrase YHVH Adonai as GOD my Lord. Whenever YHVH and Adonai are used together, the practice of capitalization is to render YHVH as GOD. The literal Hebrew is simply Jehovah Adonai that is Jehovah Lord.

However, the NWT sees the problem with Jehovah Lord and adds words completely foreign to the actual Hebrew (or Greek), But you are Jehovah the Sovereign Lord... Thus, a clear expression of the Psalmist calling Jehovah, Lord asking for mercy, is distorted into a declaration of Jehovah Witness doctrine: But you are Jehovah, the Sovereign Lord.