a

音标发音

通常來說，a 是用在以輔音開首的詞前麪，an 是用在以元音開首的詞前麪；
可數名詞前麪用 a（或 an），不可數名詞前麪不可用a（或 an)，如可以說 a healthy boy，卻不可以說 a health；
在特定情況下，a（或 an）可以用在複數形式前麪:習語中有a few (books)，a great many (books)等表示方法。在表示時間或款項的短語前麪，如 a further five minutes。在若乾在形式上是複數而在意思上是單數的單詞前麪，如 a barracks，a links。在美國口語中，如 a good ways，a long ways；
a（或 an）有時有“一個而不是那所要的一個”的意思。a 也有“任何一個”的意思。I got a letter indeed，but it was from a friend. 事實上，我收到了一封信，但衹是一個朋友的來信。I wanted to read a book，I did not care whether it was a history，a novel，or a biography.
a 與 few 搭配使用時，few 是“很少”，有“幾乎沒有”的意思。a few 是“若乾”，有“頗有幾個”的意思。注意，only a few 卻等於 few，也是“很少”的意思。

(NOT PARTICULAR) used before a noun to refer to a single thing or person that has not been mentioned before, especially when you are not referring to a particular thing or person
(ONE) one

You use a or an when you are referring to someone or something for the first time or when people may not know which particular person or thing you are talking about.
You use a or an when you are referring to any person or thing of a particular type and do not want to be specific.
You use a or an in front of an uncount noun when that noun follows an adjective, or when the noun is followed by words that describe it more fully.
You use a or an in front of a mass noun when you want to refer to a single type or make of something.
You use a in quantifiers such as a lot, a little, and a bit. (a lot), (a little), (a bit##bit)
You use a or an to refer to someone or something as a typical member of a group, class, or type.
You use a or an in front of the names of days, months, or festivals when you are referring to one particular instance of that day, month, or festival.
You use a or an when you are saying what someone is or what job they have.
You use a or an instead of the number "one," especially with words of measurement such as "hundred," "hour," and "metre," and with fractions such as "half," "quarter," and "third." (one)
You use a or an in expressions such as eight hours a day to express a rate or ratio.
&rarran informal or dialect word for have
&rarr(usually linked to the preceding noun) an informal form of of
acceleration