a

一、一個用於可數名詞單數前

She adopted a cat from the shelter last month.

她上個月從收容所領養了一隻貓。

每

He goes swimming three times a week.

他每週去游泳三次。

某個、某位

A Mr. Lin called while you were out.

你外出時有位林先生打過電話來。

一種、一類表示種類

This is a cheese I have never tasted before.

這是一種我以前沒吃過的乳酪。

字母A

The word "apple" begins with a lowercase a.

apple 這個詞以小寫字母 a 開頭。