(Sorry to revert to English!) Would the above sentence still make sense if 'la' was inserted before 'casa'? It would then be consistent with the other similar examples, which would make it easier for me to remember.
Gracias y saludos,
It wouldn't be completely incorrect to say "estoy en la casa" but it's not natural at all to use the article. It is a set expression where the article is dropped, the same way it'd be odd/incorrect to say in English "I am at the home". You simply say I'm home or I'm at home (no articles here either)
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