Units first: einundzwanzig

German says the small number first — ones + und + tens, all in one word:

• 21 = einundzwanzig, literally “one-and-twenty”
• 34 = vierunddreißig — the vier comes first even though you write the 3 first
• 56 = sechsundfünfzig — here sechs keeps its s

Note ein, not eins, in einundzwanzig. This backwards habit costs real Hören points, so drill it until it feels normal.