I feel like I've seen him somewhere before. Where was it?
I swear I've seen his face somewhere before. I can't remember where it was though.
この場合、次のような言い方ができますよ。
ーI feel like I've seen him somewhere before. Where was it?
「前に彼のことをどこかで見たことがある気がする」
ーI swear I've seen his face somewhere before. I can't remember where it was though.
「前に絶対どこかで彼の顔を見たはず。どこだったか思い出せないけど」
ご参考まで！
カナダ 
