In this verse the people recall the awesome things God did in the past for their ancestors, so that he will do the same things for them.

When thou didst terrible things which we looked not for: The Hebrew word rendered terrible things comes from a root that can mean “to fear,” but more often it means “to be in awe” (see the comments on 41.5, where it is translated “are afraid”). Here it refers to events that are awesome rather than fearful. Good News Translation says “terrifying things,” but better renderings are “awesome deeds/things” (New Revised Standard Version, New International Version, Revised English Bible) and “wonders” (New Jewish Publication Society’s Tanakh). In the context of this lament the terrible things are probably the events leading to the release of the Israelites from slavery in Egypt, as well as the events during the Exodus itself. These events were not catastrophes for them, but helped them greatly. Which we looked not for means the awesome events were completely unexpected (so Good News Translation, New International Version), beyond the imagination of the people. For this whole line Revised English Bible has “You surprised us with awesome things.”

Thou camest down, the mountains quaked at thy presence recalls what happened when God came to the Israelites at Mount Sinai during the Exodus (see Exo 19.18). This line basically repeats the last half of verse 1, so some scholars have suggested it is a scribal error. However, Hebrew Old Testament Text Project supports keeping it.

Like Revised Standard Version, most versions use past tense for the Hebrew expression for When thou didst, but it could be rendered with present or future tense. If this is done, then the whole verse is continuing the plea for God to come down and make the mountains shake. De~Waard allows for this possible interpretation. A model for the verse that follows this sense is “When/If you come down and perform awesome deeds that we cannot imagine, may the mountains shake in your presence” (similarly Bible en français courant; see also the second example).

Translation examples for this verse are:

• When you performed amazing things that we could not have expected,
you came down and the mountains shook in your presence.

• When you perform awesome things that we cannot anticipate,
come down and make the mountains tremble before you.

Quoted with permission from Ogden, Graham S. and Sterk, Jan. A Handbook on Isaiah. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here .