Questions & Answers about El corazón es un órgano muy importante.
Would I use corazón if I want to say I learned facts about this organ 'by heart'?
No, the English idiom 'by heart' doesn't use the word corazón. Instead, you use de memoria (literally 'by memory'). For example, Aprendí la lección de memoria. The word corazón is used for the physical organ or emotional feelings, but never for memorization.
If I make corazón plural, does it keep the accent mark on the 'o'?
No, the plural is corazones without the accent mark. Words ending in -n or -s with an accent on the final syllable drop that written accent in the plural. Adding -es adds a new syllable, making the natural spoken stress fall on the second-to-last syllable (-zo-), so the written mark is no longer needed.
Could I ever use está instead of es if I were describing the heart's temporary condition as an organ?
No, you must always use ser (here as es) when equating a subject with a noun phrase like un órgano. Even though you might think of temporary states with body parts, estar can never be used to link a subject directly to a classifying noun.