Questions & Answers about O terceiro está bem.
Because terceiro is functioning like a noun phrase: o terceiro = the third one / the third person / the third item, depending on context.
In Portuguese, ordinal numbers like primeiro, segundo, terceiro can be used this way with a definite article:
- o primeiro = the first one
- o segundo = the second one
- o terceiro = the third one
So o is not random—it marks it as the specific third one.
Because terceiro is the masculine singular form.
Ordinal numbers in Portuguese agree in gender and number with the noun they describe or replace:
- o terceiro = the third one/item/person (masculine)
- a terceira = the third one/item/person (feminine)
For example:
- O terceiro livro = the third book
- A terceira cadeira = the third chair
If the thing being referred to is feminine, you would say A terceira está bem.