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.
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.
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，我们为何没有threelif, fourlif, fiflif和sixlif 呢？答案跟历史中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.
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.
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.