These are articles. They are used in front of nouns (things)
There are three categories:
A / an = when the object is countable and is only one( example. A table, a door, an apple) this is used mainly when it's not identified which one. It could be any! If I say
"I saw a man yesterday" it could be any man. I'm not referring to which man I saw, it could have been anyone...
This article is called "indefinite". Because it does not define anything.
The = this article is used when a specific thing is being referred, or an ONLY thing is being talked about. Example. The sun, or the moon ( there's only one of those close to our planet) the President, the Prime Minister, etc
This also refers to something that has already been identified. If I say
"I saw the man yesterday" it's because we already know who this man is. And if you don't know, you'll immediately say "Which man?!?"
This article"the" can be used with countable and non countable nouns, plural and singular.
The water, the door, the people, the tables, the dogs that eat grass near the park where people go running.....etc
O = There's another article. It's referred to as zero article. It's when you use none instead.
"I saw John yesterday"
"I'd like to have brothers"
No articles are needed there
Hope this helps. If you have any other questions or doubts you can contact me.