YOU SAID:
He promised me that he would come back soon.
INTO JAPANESE
彼はすぐに戻ってくると私に約束した。
BACK INTO ENGLISH
He promised me he would be back soon.
INTO JAPANESE
彼はすぐに戻ってくると私に約束した。
BACK INTO ENGLISH
He promised me he would be back soon.
That didn't even make that much sense in English.