Numbers and the clock
Two famous quirks await you here. (1) From 21 onward German reads the digits right-to-left: einundzwanzig = literally 'one-and-twenty' = 21. (2) halb drei literally says 'half three', but it means 2:30 — half of the way to three, not half past three. Make peace with both now and the rest is mechanical.
0–12: just memorize
null (0), eins (1), zwei (2), drei (3), vier (4), fünf (5), sechs (6), sieben (7), acht (8), neun (9), zehn (10), elf (11), zwölf (12). eins is the counting form ('one'); when it sits before a noun it loses the -s and becomes ein / eine, exactly like the indefinite article: ein Buch, eine Frau, but Ich habe eins. = 'I have one.'
13–19: digit + zehn
dreizehn (13), vierzehn (14), fünfzehn (15), sechzehn (16, note: drop the -s of sechs), siebzehn (17, drop the -en of sieben), achtzehn (18), neunzehn (19). Pattern: digit + zehn, with two small contractions at 16 and 17.
20–100: tens, then digits backwards
Tens: zwanzig (20), dreißig (30 — note -ßig, not -zig), vierzig (40), fünfzig (50), sechzig (60), siebzig (70), achtzig (80), neunzig (90), hundert (100). Compound numbers read digit + und + tens, written as one word: einundzwanzig (21), zweiundzwanzig (22), dreiundzwanzig (23), vierundvierzig (44), siebenundsechzig (67), neunundneunzig (99). Yes, 47 is siebenundvierzig — 'seven-and-forty'. Train your ear: native speakers say it fast and you have to flip the digits in your head.
Telling time — Wie spät ist es?
Asking: Wie spät ist es? or Wie viel Uhr ist es? = 'What time is it?'. Whole hours: Es ist drei Uhr. = 'It's three o'clock.' Quarter past: Viertel nach drei = 'a quarter past three' = 3:15. Quarter to: Viertel vor vier = 'a quarter to four' = 3:45. The trap: halb drei literally = 'half three' but it means 2:30 — German thinks of it as 'halfway to three', not 'half past two'. So halb vier = 3:30, halb fünf = 4:30. To say the minutes: zehn nach drei (3:10), zwanzig vor vier (3:40).
am / pm with um and Uhr
um = 'at' (with clock times). Um drei Uhr. = 'At three o'clock.' To clarify morning vs evening, add morgens (in the morning), nachmittags (in the afternoon), abends (in the evening), nachts (at night). Um sieben Uhr abends. = 'At 7 p.m.' In writing and timetables German just uses 24-hour: 19 Uhr = 7 p.m.
