In English,
Definite article: the
Indefinite article: a, an
If a noun is preceded by ‘ஒரு’ or ‘ஒர்’ you would always translate it as the indefinite article “a”/ “an”. ஒர் is used when the following word begins with a vowel.
There is no Tamil equivalent for the definite article “the”. Instead we use the demonstrative articles ‘அது’ / ‘இது’ (“this” or “that”).
A good rule to use is when translating if a noun is not preceded by ‘ஒரு’, ‘ஒர்’ , ‘அது’ or ‘இது’ is try to translate it with the definite article “the” unless the context makes it clear that we are talking about an unknown noun (in which case you would translate using “a” / “an”).
For more instruction on the articles: