They came back home because they forgot the book.
"Forgot" is in the simple past tense because the action of forgetting the book occurred before the action of coming back home. The verb "come" is also in the simple past tense to match the tense of the main verb "forgot."
They came back home because they forgot the book.
"Forgot" is in the simple past tense because the action of forgetting the book occurred before the action of coming back home. The verb "come" is also in the simple past tense to match the tense of the main verb "forgot."