YOU SAID:
It was early in the morning when I heard it.
INTO JAPANESE
私がそれを聞いたのは朝早かった。
BACK INTO ENGLISH
It was early in the morning that I heard it.
INTO JAPANESE
私がそれを聞いたのは朝早くでした。
BACK INTO ENGLISH
It was early in the morning that I heard it.
That didn't even make that much sense in English.