In English, nouns can have both definite articles (the ) and indefinite article (a or an ).
In Gaelic, we only use the definite article. We don't use indefinite articles. So for example, there is no difference in Gaelic between apple and an apple - they are both translated as ubhal :)
In English it's unidiomatic to not use an article with singular countable nouns. So when there's no article in Gaelic you always have to "add" an indefinite article when translating into English provided it's a singular countable noun.
Tha mi ag ithe ubhal. I'm eating an apple.
Tha mi ag ithe an t-ubhal. I'm eating the apple.
Tha mi ag ithe rìs. I'm eating rice. (uncountable)