Articles “a/an” and “the” are what is called basic grammar – but basic doesn’t mean easy.
English is a very exact language and that means that all nouns need a ‘determiner’ in front of them. Articles are a type of determiner. They give us information about the noun.
“The” is called the definite article. It comes before words the listener/ reader already knows. “Hey honey, the cat is on the sofa” could be a husband talking to his wife about their pet.
“A/an” is the indefinite article, it is used to describe any member of a group. “Hey honey, a cat is on the sofa” would be the husband telling the wife that one random cat out of all the cats in the world is on the sofa. This could be a problem, no one wants a random cat on their sofa.
Nouns can quickly change from indefinite to definite.
Sentence 1: “A man walked into the restaurant.” indefinite: we only know that he is one of the many men in the world. His identity is unknown or unimportant.
Sentence 2: “Then the man sat down.” definite: we are talking about the man introduced in the previous sentence. We still don’t know who he is, but we know he is the same man as before.
If it was Sentence 1: “A man walked into the restaurant.” then Sentence 2: “Then a man sat down.” We would be saying one man in the world walked into a restaurant and one of the men in the world sat down. There is nothing indicating they are the same.