English grammar rule The Superlative

Unique nouns are indicated by using the definite article the. Nouns become unique when modified with a superlative (best, worst) or an ordinal number (tenth, hundredth). There are other adjectives that can also make a noun unique depending on context (“The white house” when others around it are blue.)

Examples of 'The Superlative' rule

Incorrect	They had a reception for an champion runner.
Correct	They had a reception for the champion runner.
Incorrect	The new teacher will start on a second day of the school year.
Correct	The new teacher will start on the second day of the school year.
Incorrect	That was a final straw for the principal.
Correct	That was the final straw for the principal.