If you think I am always sleeping, you are very wrong!
If you think I am always sleeping, you are very much mistaken.
「私がいつも寝ていると思ったら大間違いだ」を英語に訳したいならこのようになります。
If you think I am always sleeping, you are very wrong!
If you think I am always sleeping, you are very much mistaken.
例文
If you think I am always eating healthy, you are very much mistaken.
私がいつも健康な食べ物を食べているばかりと思ったら大間違えだ。
If you think I am fluent in Spanish, you are very wrong.
私がスペイン語がペラペラと思ったら、大間違えだ。
If you think I am always on time, I am sorry to say, but you are very mistaken.
私がいつも間に合える人と思ってら、残念ながら、大間違えだ。
ご参考までに!