It's hard to understand exactly how another person is feeling.
It's difficult to understand other people's feelings 100%.
ーIt's hard to understand exactly how another person is feeling.
「相手の気持ちを正確に理解するのは難しい」
It's hard to understand で「理解するのが難しい」
exactly「正確に・厳密に」を使って100%を表現することができます。
ーIt's difficult to understand other people's feelings 100%.
「相手の気持ちを100%理解するのは難しい」
It's difficult to understand で「理解するのが難しい」
to understand other people's feelings 100% で「他の人の気持ちを100%理解する」
ご参考まで!