a or the

[Rehabissa]

Hellow
I'm Rehab from Syria pleas help me
I don't know how can I use a or the or nothing befor some words

[ollieacappella]

You use "a"/"an" if you are not talking about a specific thing and "the" if you are. For example, saying "I want an apple" means "any apple would make me happy", whereas "I want the apple" means that you want the apple which is in front of you, or the apple which you have been speaking about already.

In "I saw a fox", it doesn't matter which fox you saw - the point is that you saw a fox.
In "I saw the fox", you mean a specific fox which you had spoken about before.