Moses is speaking to the people.

You came near: that is, near to where Yahweh was.

Stood at the foot of the mountain: Mount Sinai (see Exo 19.17).

The mountain burned with fire to the heart of heaven: this sounds like the description of a volcanic eruption, with the flames leaping high in the sky. New Jerusalem Bible has “the mountain flamed to the very sky,” New Revised Standard Version translates “the mountain was blazing up to the very heavens,” and Contemporary English Version has “the fire went up to the sky.”

The mountain was wrapped in darkness, cloud, and gloom (see Exo 19.18); the Hebrew text says “… the heart of heaven, darkness, cloud, and gloom” (without a verb). Some translations do not take the three words to represent three different phenomena, but a way of saying “dark cloud and thick mist” (Revised English Bible), “black clouds and deep darkness” (New International Version), “deep dark clouds” (Contemporary English Version). The Hebrew word translated gloom is translated in the Septuagint by a Greek word that means “storm,” or “whirlwind” (see its use in Heb 12.18). The Septuagint adds, at the end, “a loud voice” (see 5.22), but this should not appear in the translation.

Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on Deuteronomy. (UBS Helps for Translators). New York: UBS, 2000. For this and other handbooks for translators see here .