le robe vs les robes
How can you tell the difference is pronunciations?
Well, first, before I answer your question, I should correct something. "Robe" is feminine, so it would be "la" robe. There is a bigger difference in pronunciation between "la" and "les" than there is between "le" and "les."
After you spend some time with the French language, this will become easier. You have to learn to pay very close attention to the articles "le," "la," and "les" because "robe" and "robes" are pronounced EXACTLY the same.
So, as you're listening, you hear... [blah blah blah blah] LA "roab" (my English pronunciation spelling for the French "robe") [blah blah blah]
[blah blah blah blah] LES "roab" (my English pronunciation spelling for the French "robes") [blah blah blah]
and you have to develop the ability to pick up on this to figure out the context.
Hope this helps.