As far as I can tell after going through a few words in my head, I put a soft "s" (as in "sit") after a soft "th" (as in "tenths"), and a hard "s" (like "z") after a hard "th" (as in "lathes"). In some words I change from a soft "th" to a hard "th" when I make the plural -- as with "path" (soft) "paths" (hard).
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