It's hard to believe he's sick because he still seesms so energetic.
I can't believe he's sick, he doesn't look like it at all.
ーIt's hard to believe he's sick because he still seems so energetic.
「すごく元気そうで病気だなんて信じられない」
It's hard to believe ... で「…と信じられない・嘘のようだ」
energetic で「元気な・活気に満ちた」
ーI can't believe he's sick, he doesn't look like it at all.
「彼が病気だなんて信じられない、全然(病気に)見えないわ」
I can't believe ... で「…と信じられない・目を疑う」
doesn't look like it at all で「全然そのように見えない」
ご参考まで!