This is a really interesting question. In Chinese number names, and both Korean number systems, the numbers follow strict base 10 representations: the name for any number from 11-19 is the name for 10 plus the name for the unit value. e.g. the Chinese name for 14 - 十四, "Shí sì" - can be literally translated as "10-4". But this strict regularity doesn't seem to exist in the teens of most European languages: In English (as you have noticed) and German, the words for 11 and 12 have unpredictable names only historically related to 1 and 2. In most Romance languages there is some phonetic representation of 10 in the names for teen numbers (albeit heavily modified in Spanish and French), but this representation 'flips' somewhere along the way (between 15-16 in Spanish, between 16-17 in Italian and French etc). See below: Units English | Latin | Spanish | Portuguese | Italian | French | zero | nihil | cero | zero | zero | zéro | one | ūnus | uno | um | uno | un | two | duo | dos | dois | due | deux | three | trēs | tres | três | tre | trois | four | quattuor | cuatro | quatro | quattro | quatre | five | quīnque | cinco | cinco | cinque | cinq | six | sex | seis | seis | sei | six | seven | septem | siete | sete | sette | sept | eight | octō | ocho | oito | otto | huit | nine | novem | nueve | nove | nove | neuf | ten | decem | diez | dez | dieci | dix Teens English | Latin | Spanish | Portuguese | Italian | French | eleven | ūndecim | once | onze | undici | onze | twelve | duodēcim | doce | doze | dodici | douze | thirteen | trēdecim | trece | treze | tredici | treize | fourteen | quattuordecim | catorce | catorze | quattordici | quatorze | fifteen | quīndecim | quince | quinze | quindici | quinze | sixteen | sēdecim | diez y seis | dezesseis | sedici | seize | seventeen | septendecim | diez y siete | dezessete | diciassette | dix-sept | eighteen | duodēvīgintī | diez y ocho | dezoito | diciotto | dix-huit | nineteen | ūndēvīgintī | diez y nueve | dezenove | diciannove | dix-neuf | twenty | vīgintī | veinte | vinte | venti | vingt Classical Latin itself is regular up until 18 and 19, where representations translated as "2 from 20" and "1 from 20" are the norm (if this seems unexpected, remember the roman numeral representation for 19 is XIX). While it might seem strange that the 'flip' in the Romance languages representations wasn't present in Classical Latin, switched forms such as "decem et septem" for 17 can be found in some Latin texts, and was a feature of some rural and lower class Latin speech that was presumably particularly prevalent among the settlers and soldiers, whose speech directly affected modern Romance languages. Source: The Nature of Mathematical Thinking , Robert J. Sternberg, Talia Ben-Zeev More on reddit.com

You could try Duolingo. It's free and pretty decent.

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