Why were Adam and Eve commanded to replenish instead of plenish? You can’t replenish the earth unless it had already been plenished. Mortals before Adam and Eve?
Those first commandments in the Garden of Eden have no reference to pre-Adamic man. Joseph Fielding Smith addressed this question many years ago.
“It is true that the original meaning of the word replenish connotes something is being filled again that was once filled before: Re—again, plenus—full. Why the translators of the King James Version of the Bible used the word replenish may not be clearly known, but it is not the word used in other translations and is not the correct meaning of the Hebrew word from which the translation was originally taken. It is true that the Prophet Joseph Smith followed the King James Version in the use of this word, perhaps because it had obtained common usage among the English-speaking peoples. Replenish, however, is incorrectly used in the King James translation. The Hebrew verb is Mole [pronounced Mah-lay] … meaning fill, to fill, or make full. This word Mole is the same word which is translated fill in Genesis 1:22, in the King James Bible, wherein reference is made to the fish, fowl, and beasts of the earth.” (Answers to Gospel Questions, 1:208–9.)
I would add that Strong’s Concordance includes among the meanings of the term “fulfill”. Consider how well Adam’s commandments latter-day revelation. He is to 1) be fruitful and multiply, 2) fill and fulfill, and 3) subdue the earth as his dominion (Gen 1:28). The Lord has testified that the earth will fill “the measure of its creation” after it has been “sanctified from all unrighteousness” and “prepared for the celestial glory” so that celestial heirs may inherit it (D&C 88:18-20). Adam and his children have been commanded to aid this earth in meeting her destiny.