How to write ordinal numbers (those denoting order) down in words?

Most ordinal numbers end in "th", so just add "th" to the end of the cardinal number (those denoting quantity, ex: 1 one, 2 two, 3 three, 4 four, ... etc.):

• cardinal: 4 = four -> ordinal: 4th = fourth
• cardinal: 7 = seven -> ordinal: 7th = seventh
• cardinal: 10 = ten -> ordinal: 10th = tenth
• cardinal: 19 = nineteen -> ordinal: 19th = nineteenth
• cardinal: 100 = one hundred -> ordinal: 100th = (one) hundredth
• cardinal: 1,000 = one thousand -> ordinal: 1,000th = (one) thousandth

Exceptions:

• cardinal: 1 = one -> ordinal: first = 1st
• cardinal: 2 = two -> ordinal: second = 2nd
• cardinal: 3 = three -> ordinal: third = 3rd
• cardinal: 5 = five -> ordinal: fifth = 5th
• cardinal: 8 = eight -> ordinal: eighth = 8th
• cardinal: 9 = nine -> ordinal: ninth = 9th
• cardinal: 12 = twelve -> ordinal: twelfth = 12th
• cardinal: 20 = twenty -> ordinal: twentieth = 20th
• cardinal: 30 = thirty -> ordinal: thirtieth = 30th
• cardinal: 40 = forty -> ordinal: fortieth = 40th
• cardinal: 50 = fifty -> ordinal: fiftieth = 50th
• cardinal: 60 = sixty -> ordinal: sixtieth = 60th
• cardinal: 70 = seventy -> ordinal: seventieth = 70th
• cardinal: 80 = eighty -> ordinal: eightieth = 80th
• cardinal: 90 = ninety -> ordinal: ninetieth = 90th

Full article: how to write ordinal numbers out in (US) American English words, using letters instead of numerals

Here there are a couple of the main ordinal numbers, from 1st up to 19th, you can find a longer list in the full article:

• cardinal: 1 = one;
  ordinal: 1st = first;
• cardinal: 2 = two;
  ordinal: 2nd, second;
• cardinal: 3 = three;
  ordinal: 3rd, third;
• cardinal: 4 = four;
  ordinal: 4th, fourth;
• cardinal: 5 = five;
  ordinal: 5th, fifth;
• cardinal: 6 = six;
  ordinal: 6th, sixth;
• cardinal: 7 = seven;
  ordinal: 7th, seventh;
• cardinal: 8 = eight;
  ordinal: 8th, eighth;
• cardinal: 9 = nine;
  ordinal: 9th, ninth;
• cardinal: 10 = ten;
  ordinal: 10th, tenth;
• cardinal: 11 = eleven;
  ordinal: 11th, eleventh;
• cardinal: 12 = twelve;
  ordinal: 12th, twelfth;
• cardinal: 13 = thirteen;
  ordinal: thirteenth;
• cardinal: 14 = fourteen;
  ordinal: 14th, fourteenth;
• cardinal: 15 = fifteen;
  ordinal: 15th, fifteenth;
• cardinal: 16 = sixteen;
  ordinal: 16th, sixteenth;
• cardinal: 17 = seventeen;
  ordinal: 17th, seventeenth;
• cardinal: 18 = eighteen;
  ordinal: 18th, eighteenth;
• cardinal: 19 = nineteen;
  ordinal: 19th, nineteenth;

Full article: how to write ordinal numbers out in (US) American English words, using letters instead of numerals