Carta vs. misiva
While both carta and misiva can be translated as letter in the sense of a written message, they are not interchangeable. The primary difference lies in their level of formality and commonality, with carta being the everyday term and misiva being a much more formal and literary alternative.
Carta
A1This is the most common and versatile word for letter in Spanish. It is used in all contexts, from informal notes to friends and family to formal business or official correspondence. It's the standard, neutral term you would use in everyday speech.
Recibí una carta de mi abuela desde Italia.
(I received a letter from my grandmother from Italy.)
El jefe me pidió que redactara una carta de recomendación.
(My boss asked me to write a letter of recommendation.)
Le escribió una larga carta de amor para su aniversario.
(He wrote her a long love letter for their anniversary.)
Voy a enviar una carta al ayuntamiento para quejarme del ruido.
(I am going to send a letter to the city hall to complain about the noise.)
En el museo vimos una carta escrita por un famoso general.
(At the museum, we saw a letter written by a famous general.)
Misiva
C1This is a formal, literary, and somewhat dated term for letter. It is typically reserved for official, diplomatic, or very solemn communications. Using misiva in a casual context would sound overly dramatic or pretentious. It emphasizes the official nature of the document, much like missive or dispatch in English.
El embajador entregó la misiva oficial al ministro de asuntos exteriores.
(The ambassador delivered the official missive to the minister of foreign affairs.)
El papa envió una misiva a todos los obispos del mundo.
(The pope sent a missive to all the bishops of the world.)
El abogado del acusado presentó una misiva al juez solicitando clemencia.
(The defendant's lawyer presented a missive to the judge requesting clemency.)
La junta directiva comunicó su decisión final a través de una misiva interna.
(The board of directors communicated its final decision through an internal missive.)
La novela histórica describía el intercambio de misivas entre los dos monarcas.
(The historical novel described the exchange of missives between the two monarchs.)
Summary
In short, use carta for any type of letter in everyday conversation and most written contexts. Reserve misiva for highly formal, official, diplomatic, or literary situations where you want to convey a sense of gravity and importance, much like using missive or official dispatch in English.