YOU SAID:
i SAW HIM PLAY HIS PIANO YESTERDAY
INTO JAPANESE
私は昨日彼が彼のピアノを弾くのを見ました
BACK INTO ENGLISH
I saw him play his piano yesterday
INTO JAPANESE
私は彼が昨日彼のピアノを弾くのを見ました
BACK INTO ENGLISH
I saw him play his piano yesterday
That didn't even make that much sense in English.