Carta vs. missiva
In Portuguese, both carta and missiva can be translated as letter. However, they are not interchangeable. The main difference lies in their register and commonality, with carta being the standard, everyday term and missiva being a much more formal and literary alternative.
Carta
A1This is the most common and versatile word for letter. It is used in all contexts, from informal personal notes to formal business correspondence. Carta is the default word you would use in nearly every situation. It can also have other meanings, such as playing cards or a menu/list.
Recebi uma carta da minha avó ontem.
(I received a letter from my grandmother yesterday.)
A empresa vai enviar uma carta oficial com a proposta de emprego.
(The company will send an official letter with the job offer.)
Ele escreveu uma carta aberta para o jornal local.
(He wrote an open letter to the local newspaper.)
Você pode me passar a carta de vinhos, por favor?
(Can you pass me the wine list, please?)
Nós jogamos um jogo de cartas para passar o tempo.
(We played a game of cards to pass the time.)
Ela guardou cada carta de amor que ele lhe enviou.
(She kept every love letter he sent her.)
Missiva
C1This is a formal, elevated, and somewhat archaic word for letter. It is typically used in literary, historical, or very official contexts to add a sense of solemnity or importance to the written message. You would almost never hear missiva in a casual, everyday conversation.
O historiador encontrou uma missiva rara escrita pelo imperador.
(The historian found a rare missive written by the emperor.)
A missiva diplomática selou o acordo de paz entre as nações.
(The diplomatic missive sealed the peace treaty between the nations.)
No romance, os amantes trocavam missivas secretas ao luar.
(In the novel, the lovers exchanged secret missives by moonlight.)
O advogado apresentou a missiva como prova irrefutável no tribunal.
(The lawyer presented the missive as irrefutable evidence in court.)
A leitura da missiva papal foi um evento de grande importância.
(The reading of the papal missive was an event of great importance.)
O general enviou uma missiva ao seu soberano descrevendo a vitória.
(The general sent a missive to his sovereign describing the victory.)
Summary
In short, carta is the standard, all-purpose word for a letter that you should use in 99% of situations, covering everything from personal notes to business mail. Missiva is a formal and literary term reserved for contexts where you want to convey a sense of historical weight, officiality, or solemnity, making it unsuitable for everyday communication.