how do you know to use "les" instead of "des"

Bonjour Judith,

Here is a link that might help you: 

Plurals of the and a = les and des (articles) in French

Also, look at the following links:

Plurals of the and a = les and des (articles) in French

Plurals of the and a = les and des (articles) in French

Plurals of the and a = les and des (articles) in French

Plurals of the and a = les and des (articles) in French

I hope this is helpful.

Bonne journée !