en_tn_lite_do_not_use/mat/13/35.md

what had been said through the prophet might come true, when he said

This can be stated in active form. Alternate translation: "what God told one of the prophets to write long ago might come true" (See: rc://en/ta/man/translate/figs-activepassive)

when he said

"when the prophet said"

I will open my mouth

This is an idiom that means to speak. Alternate translation: "I will speak" (See: rc://en/ta/man/translate/figs-idiom)

things that were hidden

This can be stated in active form. Alternate translation: "things that God has kept hidden" (See: rc://en/ta/man/translate/figs-activepassive)

from the foundation of the world

"since the beginning of the world" or "since God created the world"