In order to make our knowledge of the occurrences in question effective, we will start with very practical examples:
During the meeting there was a official speaking out about the strike.
Only a official spoke about the strike during the meeting.
Common situations put us in check, especially when it comes to syntactic and morphological analysis, as the examples themselves point us, these are two identical words, which are incumbent on us to find out if they are the same functions now performed.
Regarding the first statement, we found that the sender's intention was to indicate the species of being, that is, it is an employee (expressed in the male gender), not an employee.
For this reason, we claim that it is a indefinite article.
In the second, the idea of quantity becomes evident, especially because the highlighted term is accompanied by the term “only”. From this perspective, when the terms "only one" or "only one" appear in the sentence, the classification that we should attribute to the word "one" will always be that of numeral.
Once the differences are found, there is no morphological analysis that tends to show any misunderstandings, as well as there is no linguistic competence that is not improved, in order to make constant use of the knowledge here acquired.
By Vânia Duarte
Graduated in Letters
Source: Brazil School - https://brasilescola.uol.com.br/gramatica/um-numeral-ou-um-artigo.htm