For is not necessary; it only represents the usual conjunction waw. The face of the whole land is really “the eye of the whole land” (see verse 5), so New Jewish Publication Society’s Tanakh translates “They hid all the land from view.” But the word for “eye” may also mean “appearance.” New Revised Standard Version has changed face to “surface,” along with others (New American Bible, Revised English Bible, New Jerusalem Bible). So that the land was darkened suggests that the locusts were in the air, blocking out the sunlight. The context, however, suggests they were on the ground, even though the word really means “dark.” Hence one may say “They covered the ground until it was black with them” (Good News Translation), or “They covered the ground so that it looked black.”

They ate all the plants in the land uses the same word as in verse 12. The locusts probably ate all the fruit off the trees and the leaves, new buds, and green sprigs off both the trees and plants. But they did not eat the larger tree branches. So and they ate all the plants … may be rendered as “They ate the greenery off all the plants and trees that the hail had not destroyed, and also the fruit on the trees.” And all the fruit of the trees is added here for emphasis; it was not part of the warning given in verse 5. Which the hail had left refers to the trees that had survived the hailstorm (9.25.) Not a green thing remained refers to any green plant life, such as new buds or sprigs that may have appeared. One may also express this as “None of the trees or plants had any greenery [or, green things] left on them.” Tree and plant of the field repeat the same words used in 9.25. This time the destruction of the plant life was complete.

Quoted with permission from Osborn, Noel D. and Hatton, Howard A. A Handbook on Exodus. (UBS Helps for Translators). New York: UBS, 1999. For this and other handbooks for translators see here .