You shall not be in dread of them: the same verb is used in 1.29 (see the comment there). Them refers to the peoples of Canaan, the enemies of the Israelites.

In the midst of you: see 6.15.

A great and terrible God: great can mean many things, depending on the context. In this verse it means that God is all powerful, and no other god can compare with him. In cultures where kings or high chiefs still rule, there will be natural expressions meaning “majestic,” “above the heads of all,” and so on. These will fit this context very well. However, in other languages it will often be necessary to use two or more terms; for example, “God is all-powerful and above every other god.” The word terrible means here “awesome” (New Revised Standard Version, New Jewish Publication Society’s Tanakh, New International Version); it is derived from the Hebrew verb “to fear,” and the meaning of this passive form is “one who is to be feared” or “one whom you should fear [or, be afraid of].”

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 .