letters
If you write a letter to someone, you write a message on paper and send it to them, usually by post.
I had received a letter from a very close friend.
我收到了一位非常亲近的朋友的来信。
a letter of resignation.
一封辞职信。
Letters are written symbols which represent one of the sounds in a language.
the letters of the alphabet.
字母表中的字母。