These fall into the category of Articles. Le, la, les are definite articles and un,une,des indefinite articles.
They tend to be required in French maybe not so in other languages,
As Chris has pointed out, this aspect of French grammar is too extensive to be dealt with in a Forum setting.
It is very key for your understanding of French grammar to study and practise this aspect of the French language thoroughly.
Then, having done so; use the forum for any specific queries after you have carried out the groundwork beforehand.
I think you need to work through the lessons. The question you ask is too broad for me to answer. If you have a more specific question, please feel free to post it.
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