Ordinal numerals show the order of persons and things in a series.

With the exception of the first three (first, second, third) the ordinal numerals are formed from cardinal numerals by means of the suffix -th.

In ordinal groups, only the last member of the group takes the ordinal form: (the) sixty-fifth, (the) twenty-third. Ordinal numerals are generally used with the definite article (the first, the fifth, the tenth, etc.). Ordinal numerals may be used with the indefinite article when they do not show a definite order of persons and things in a series:

“I’ve torn simply miles and miles of the frill,” wailed a third. (Mansfield)

As a rule, ordinal numerals are used as attributes.

“No, this is my first dance,” she said. (Mansfield)
Almost immediately the band started and her second partner seemed to spring from the ceiling. (Mansfield)

But they may also be used as subject, as predicative, and as object.

Then, advancing obliquely towards us, came a fifth. (Wells) (SUBJECT)
Sooner or later, someone is going to tell you about that damned river, so I might as well be the first. (Wilson) (PREDICATIVE)
…she noted a scar on his cheek, another that peeped out from under the hair of the forehead, and a third that ran down and disappeared under the starched collar. (London) (OBJECT)

In fractional numbers, the numerator is a cardinal and the denominator is a substantivized ordinal:

two-thirds
three-sixths

Decimal fractions are read in the following way:

7.58 — seven point (decimal) five eight.