Articles: A/An/The
From: https://www.onlineenglishteacher.org/study-topics/grammar/grammar-advice/articles-a-an-the
In English, there are two types of nouns: countable and uncountable. Countable nouns are words that you can count, such as "one computer and two computers". Uncountable nouns are words that are not possible to count (or impractical to), such as "air" or "rice".
When a word is countable and singular, try to add a word in front of it, such as ("article") "a/an/the" or another word ("determiner") like "my/your" etc., depending on the context.
Words like "a/an" are used when one is speaking generally about a topic, such as: "I saw a bird."
Words like "the" are when something is specific. For instance, we could say "I saw the bird again, that I saw yesterday."
In addition, there are more rules, such as to use "the" when there is only one thing, such as "the Earth" or for some countries, which are groups of places, like "the United Kingdom" (England, Wales, Scotland and Northern Ireland).
Here are some free links to learn more about this topic:
1. English Club = "https://www.englishclub.com/grammar/adjectives-determiners-the-a-an.htm".
2. LearnEnglish.de articles = "https://www.learnenglish.de/grammar/articlestext".