RobH_en_tn/jer/48/38.md

There is mourning everywhere

The abstract noun "mourning" can be stated as a verb. Alternate translation: "There are people mourning everywhere" (See: rc://en/ta/man/translate/figs-abstractnouns)

every flat roof

"every housetop"

plazas

open public areas, such as open marketplaces

For I have destroyed

The word "I" refers to Yahweh

this is Yahweh's declaration

Yahweh speaks of himself by name to express the certainty of what he is declaring. See how you translated this in Jeremiah 1:8. Alternate translation: "this is what Yahweh has declared" or "this is what I, Yahweh, have declared" (See: rc://en/ta/man/translate/figs-123person)