One day, Bossun discovers a box of video tapes in his mother's room. The videos show his young mother, a pretty woman named Haru, and a man who looks just like him...
She must have mistaken you for 
Mamoru-san when he was a kid.
Te confundió con Mamoru cuando era niño.
Parece que te confundía con
Mamoru cuando era pequeño.
Ela deve ter te confundido com 
o Mamoru quando criança.