一 封 信 / 写 信 / 收到 信
信 (xìn) means 'letter' or 'mail,' referring to a written message sent to someone. It is one of the most common words for personal or formal correspondence in Chinese.
你的信我已经收到了,谢谢。
Nǐ de xìn wǒ yǐjīng shōudào le, xièxie.
I've already received your letter, thank you.
现在很少有人写信了,都发微信。
Xiànzài hěn shǎo yǒu rén xiě xìn le, dōu fā Wēixìn.
Very few people write letters nowadays; everyone uses WeChat.
他每个月都给妈妈写一封信。
Tā měi gè yuè dōu gěi māma xiě yì fēng xìn.
He writes a letter to his mother every month.