Count, or name an amount

Use eins when the number stands alone, for example in a telephone number. Before a noun use ein or eine: ein Schlüssel, eine Tasse. A masculine direct object is einen Schlüssel. With numbers from two, the noun is normally plural: zwei Tassen.

The unit comes first

From twenty-one, say the unit, then und, then the tens: einundzwanzig means twenty-one and zweiunddreißig means thirty-two. Write the number as one word. The unit one is ein inside these numbers, not eins. Note the shortened forms sechzehn, siebzehn, sechzig and siebzig.

Give a telephone number

Say a phone number digit by digit or in short groups. Keep every zero, including one at the beginning. You can correct a misunderstood digit with Nicht fünf. Eins. Asking for repetition is helpful, not a failure: Noch einmal, bitte.