English number words are pretty logical after a point. From twenty-one to ninety-nine, the same principle applies: the tens place followed by the units place. But the teens are different.
英文数字从某个数开始就相当符合逻辑。从21到29，运用的都是相同的理论:几十在前，后面跟着个位数字。但十几却并非如此。
Eleven and twelve come from the Old English words endleofan and twelf, which can be traced back further to a time when they were ain+lif and twa+lif.
11和12来自古英语endleofan和twelf，可以追溯到俩数字还写作ain+lif和 twa+lif的时候。
So then the question is, why don't we have threelif, fourlif, fiflif, sixlif and so on? The answer has to do with the development of number systems over history.
于是，问题来了，我们为何没没有threelif, fourlif, fiflif和sixlif 呢？答案和历史上数字体系的发展有关。
A long, long time ago, when the number words were first being formed, most people didn't have much reason to distinguish numbers above ten. In fact, some languages of primitive cultures only have number words for one, two, and many. So the basic number words up to ten formed first, then they were extended a bit with the –lif ending.
很久很久以前数字刚刚形成的时候，多数人并没有太多能耐去辨识十以上的数字。事实上，有些原始文化的语言里表示数字的词只有一、二和许多。于是乎，先有了到十的基本数字，然后稍稍拓展，以–lif 结尾。
Many number systems are based on 12 because it's divisible by the most numbers, and because you can count to 12 on one hand by using your thumb to count three knuckles on each of the other fingers. If 11 and 12 are being used more frequently, the forms for them will stick, even when another system starts to develop.
许多数字体系都是基于12的，因为12可以被多数数字约分，还因为你可以一只手仅靠大拇指，数每个指头的指节，就能数到12。要是11和12如今运用更广泛的话，其形式便会固定下来，即便有其它体系慢慢发展起来。
You can extend that idea to other number words. We have more irregularities of pronunciation in the tens (twenty, thirty, fifty instead of twoty, threety, fivety) because we've been making everyday use of those numbers for longer than we have for two hundred, three hundred, and five hundred). 。Thousand is an old word, but its original sense was "a great multitude," a non-numerically-specific, but very useful idea. The words we needed earliest, and used the most frequently are usually the most irregular.
这样思想可以类推到其它数字。几十的数字发音不规律(twenty, thirty, fifty 而不是twoty, threety, fivety)。一千是个古老的词，最初的意思是“量很大”，具体数值不详，但这一概念很有用。有些词汇我们很早就有需要、 使用频率最高，也恰是最不规则的。
So the short answer is, we created words for 11 and 12 a long time ago by calling them "one left after ten" and "two left after ten." They were more useful to us than the higher numbers, so we said them more and they became a habit that we couldn't shake.
所以，简单说来，人们很久以前创造出“十余一”和“十余二”这样的用语来表示11和12。比起更大的数字，11和12更为有用，所以我们说的也多，因为这已成为习惯难以改变。
(来源：沪江英语)
