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Just as in English, the numbers 11 and 12 in German are outliers – they aren't formed the way the other larger numbers are. For this reason, it's easiest to learn them separately.  Eleven is "elf" (elf). Twelve is "zwölf" (tsvoolf). This is a deceptively difficult word for native English speakers to pronounce. The "teens" in German are formed similarly to the way these number-words are formed in English, although English actually gets a bit more complicated.  In German, all you have to do is take the word for ten, "zehn," which you already know. On the front of "zehn," put the word from one to nine that corresponds to the second digit of the number. So, for example, 13 in German is "dreizehn" (DRIGH-tsayn). Literally, you can think of it as "three and ten," which add up to 13. All the other number-words in the teens are formed the same way, with stress always on the first syllable. The easiest way to learn to continue counting in German is to first memorize the words for the tens – 20, 30, 40, 50, 60, 70, 80, and 90. Many of these words are formed by adding "zig" to the word for the initial number, as you'll see:  Twenty is "zwanzig" (TSVAHN-tsikh). Thirty is "dreißig" (DRIGH-sikh). The "ß," called an Eszett or scharfes S (sharp S), is a unique German letter pronounced essentially like the "s" sound in "kiss" or "bless." Forty is "vierzig" (FEER-tsikh). Fifty is "fünfzig" (FUUNF-tsikh). Sixty is "sechzig" (ZEKH-tsikh). Seventy is "siebzig" (ZEEP-tsikh). Eighty is "achtzig" (AHKH-tsikh). Ninety is "neunzig" (NOYN-tsikh). Once you know how to count by tens and how to count from one to nine, you have all the building blocks you need to create virtually any other number in German.  Start with the smallest number, and work up from there to build the word. For example, 21 in German is "einundzwanzig" (IGN-oont-tsvahn-tsikh). A literal translation would be "one and twenty," as "und" is the German word for "and." All numbers up to a hundred are built this way, with the word for 100 being "hundert" (HOON-dert). The word literally means "hundred," so 100 is einhundert. As you can probably guess, the building starts again here, the same as it did with the lower numbers. Write (or say) the hundreds first, then the digits that follow. Keep in mind that there's no "und" or "and" between the hundred and the other digits. Since people often say "three hundred and eleven" in English to mean 311, you might be tempted to add it in. Since you know how to pronounce the building blocks, you know how to build and pronounce some lengthy words. For example, 535 in German is "fünfhundertfünfunddreißig."

Summary:
Learn the words for numbers 11 and 12. See how numbers 13 through 19 are formed. Practice counting by tens. Apply logic to formulate other numbers.