回答
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It's hard for other people to understand what depression is all about.
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It's difficult for people to really understand what it's like for someone to live with depression.
ーIt's hard for other people to understand what depression is all about.
「他の人にとって、うつ病とはどういうものなのかを理解することは難しい」=「他人には理解されにくい」
It's hard for other people to understand ... を使って「理解されにくい」を表現できます。
ーIt's difficult for people to really understand what it's like for someone to live with depression.
「人々にとって、うつ病を抱えて生きていくことがどういうことなのかを本当に理解するのは難しい」＝「他人には理解されにくい」
It's difficult for people to really understand ... を使っても「理解されにくい」を表現できます。
to live with ...で「…を抱えて生きる」
ご参考まで！
カナダ 
