Exactly, face is 顔(かお). But we use the word 表情(ひょうじょう) as the translation of "facial expression". And show is 見せる exactly, too. But the word "show" also includes the meaning of "teach" or "tell". 見せる doesn't have these nuances. We use the words 示す(しめす) 表す(あらわす) 表現する(ひょうげんする) in such cases. So in natural Japanese, your sentence goes: 表情ひとつで、たくさんの感情を表現することができます。 ...I didn't comment about the word 感情(かんじょう). It's rather complex. But if I try to explain, 気分 is a subjective noun and 感情 is a objective noun. And if you want to talk about "showing" something, objective words are not suitable. ...I hope you haven't got confused. Please enjoy studying.