Nuts & Bolts of German (Theory): Ordinal numbers

1 - eins	1^st - ers+t plus an ending
2 - zwei	2^nd - zwei+t plus an ending
3 - drei	3^rd - drit+t plus an ending
4 - vier	4^th- vier+t plus an ending
5 - fünf	5^th- fünf+t plus an ending
6 - sechs	5^th- sechs+t plus an ending
7 - sieben	7^th- sieb+t plus an ending (less common: sieben+t plus an ending)
8 - acht	8^th- acht+ plus an ending (no second "t" is added)
9 - neun	9^th- neun+t plus an ending
10 - zehn	10^th- zehn+t plus an ending
11 - elf	11^th- elf+t plus an ending
12 - zwölf	12^th- zwölf+t plus an ending
13 - dreizehn	13^th- dreizehn+t plus an ending
14 - vierzehn	14^th- vierzehn+t plus an ending
15 - fünfzehn	15^th- fünfzehn+t plus an ending
16 - sechzehn	16^th- sechzehn+t plus an ending
17 - siebzehn	17^th- siebzehn+t plus an ending
18 - achtzehn	18^th- achtzehn+t plus an ending
19 - neunzehn	19^th- neunzehn+t plus an ending
20 - zwanzig	20^th- zwanzig+st plus an ending
21 - einundzwanzig	21^st - einundzwanzig+st plus an ending
22 - zweiundzwanzig	22^nd - zweiundzwanzig+st plus an ending
.....	.....
30 - dreißig	30^th - dreißig+st plus an ending
31 - einunddreißig	31^st - einunddreißig+st plus an ending
.....	.....
100 - hundert	100^th - hundert+st plus an ending
.....	.....
1000 - tausend	1000^th - tausend+st plus an ending

Ordinal numbers express sequence, e.g. first, second, third, fourth etc.

Formation:

Add -t and the necessary ending to the cardinal number up to the nineteenth, from then on add -st and the necessary ending.

The irregular forms are highlighted.

Which ending do I need?

Ordinal numbers take the same endings as adjectives that precede a noun.

After "am" and "im", for example, you have to add an -en because that is the necessary dative ending (am = an+dem, im = in+dem), e.g.:

Er hat am zwölften April Geburtstag. (His birthday is on the 12th of April.)

Sie wurde am neunundzwanzigsten Februar geboren. (She was born on February 29th.)

Sie sind im vierten Semester. (They are in their 4th semester.)

Wir wohnen im fünften Stock. (We area living on the 5th floor.)

When you tell somebody today's date, the ending is -e because that is the necessary nominative ending:

Heute ist der fünfzehnte Februar. (Today is the 15th of February.)

Note: In German, you put a dot after a digit which expresses an ordinal number, e.g.

1^st = 1.
2^nd = 2.
3^rd = 3.
5^th = 5.

Nuts & Bolts of German (Explanations): Ordinal numbers (A2)