2

If the word isn’t specific, so need ’a’. Or If the word isn’t common, so need ’the’.

Which way your mental process works?

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  • 1
    Welcome to EL&U. I don't think this question can adequately be answered here, as whole chapters of grammars are devoted to the concept of definiteness and what is to be considered definite or indefinite and what determiners are used in what situations. Definiteness has nothing to do with commonality, it has to do with how the speaker expects the listener to understand a reference to a particular noun, Our sister site for English Language Learners may be of interest, or their Resources for Learning English. – choster Mar 12 '19 at 15:38
  • "This morning a woman knocked at my door with a parcel. She was not the woman who usually delivers my mail." The second woman referred to is a particular person, the first an anonymous stranger. – Kate Bunting Mar 12 '19 at 17:48
4

You use 'the' when referring to something specific. 'The' is a definite article.
'A' when referring to something general, it's an Indefinite article.

This is 'a' computer

or

'The' computer over there

One of many sources: https://www.dictionary.com/e/definite-vs-indefinite-articles/

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