an

词源同one, inch, unit.

an

an: [OE] The indefinite article in English is ultimately identical with the word one (as is the case, even more obviously, in other European languages – French un, German ein, and so on). The ancestor of both a(n) and one was ān, with a long vowel, but in the Old English period it was chiefly used for the numeral; where we would use a(n), the Anglo-Saxons tended not to use an article at all. Ān begins to emerge as the indefinite article in the middle of the 12th century, and it was not long before, in that relatively unemphatic linguistic environment, its vowel became weakened and shortened, giving an.

And at about the same time the distinction between an and a began to develop, although this was a slow process; until 1300 an was still often used before consonants, and right up to 1600 and beyond it was common before all words beginning with h, such as house.
=> one

an

indefinite article before words beginning with vowels, 12c., from Old English an (with a long vowel) "one; lone," also used as a prefix an- "single, lone;" see one for the divergence of that word from this. Also see a, of which this is the older, fuller form.

In other European languages, identity between indefinite article and the word for "one" remains explicit (as in French un, German ein, etc.) Old English got by without indefinite articles: He was a good man in Old English was he wæs god man. Circa 15c., a and an commonly were written as one word with the following noun, which contributed to the confusion over how such words as newt and umpire ought to be divided (see N).

In Shakespeare, etc., an sometimes is a contraction of as if (a usage first attested c. 1300), especially before it.

1. Beauty is an attitude. It has nothing to do with age.

美是一种态度，与年龄无关。

来自金山词霸

2. Good luck is when an opportunity comes along and you're prepared for it.

好运就是当机会来临时，你早已做好了准备。

来自金山词霸 每日一句

3. Here's an inside tip: The faster you rise, the harder you fall.

给你一个小忠告：爬得越快，摔得越疼。

来自美剧《绯闻女孩》

4. I thought I'd enrol you with an art group at the school.

我想我会吸收你参加学校的一个艺术团。

来自柯林斯例句

5. An attempt by anti-abortionists to tighten the rules was defeated.

反堕胎者所作的加强法规的尝试失败了。

来自柯林斯例句