Here Isaiah describes events that will happen in the future as if they happened in the past. This emphasizes that they certainly will happen. (See: [[rc://en/ta/man/translate/figs-pastforfuture]])
What Isaiah just found out is so amazing that he wonders if the exiles will believe it. The "we" includes him and those in exile. Alternate translation: "It is hard for anyone to believe what we have heard" (See: [[rc://en/ta/man/translate/figs-inclusive]] and [[rc://en/ta/man/translate/figs-rquestion]])
Arm refers to God's power. This can be stated in active form. Alternate translation: "Yahweh has revealed his power to people." (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-activepassive]] and [[rc://en/ta/man/translate/figs-rquestion]])