en_tn_condensed/rev/09/04.md

They were told not to damage the grass on the earth or any green plant or tree

Ordinary locusts were a terrible threat to people because when they swarm, they can eat up all the grass and all the leaves on plants and trees. These locusts were told not to do this.

but only the people

The phrase "to damage" or "to harm" is understood. Alternate translation: "but only to harm the people" (See: rc://en/ta/man/translate/figs-ellipsis)

the seal of God

The word "seal" here refers to a tool that is used to press a mark onto a wax seal. In this case the tool would be used to put a mark on God's people. See how you translated "seal" in Revelation 7:3. Alternate translation: "the marker of God" or "stamp of God" (See: rc://en/ta/man/translate/figs-metonymy)

foreheads

The forehead is the top of the face, above the eyes.