I can't hear the difference between ष and श on duolingo in the character lessons. Can others?
For what difference should I be listening?
In most actual day-to-day usage, these characters are indistinguishable from each other. Fortunately there are pretty much no words that use their distinction as a key difference in meaning, so you don't really have to worry about it. I would say that techincally the ष is supposed to be a "stronger/sharper" sound, as my Hindi teacher back in grade school used to say, but even then it's very hard to notice.
This is reposting from my response to another question in this forum:
I grew up speaking Hindi but did not learn to read/write Hindi until much later, and from my perspective, there is no difference between these sounds in Hindi. I can't tell which letter to use by sound alone.
Some very proper or traditional people will say there are differences in sound, because in Sanskrit, the sounds are indeed different. But not in Hindi, not really.
The rule that has worked for me is: almost always use श and you'll be fine. In any word of Arabic, Persian, or English origin, you want श. I also think most native Hindi words use श.
ष , the other hand, will be used only in some words of Sanskrit origin. So you will see it in names like Rishi, Santosh, Harsh, Vishnu, Krishna. You will see it in "kosh"/"shabdkosh" ("dictionary"). Various other words. You'll pick it up over time.
If you don't know, err on the side of श. Hope that helps!