What's the difference between weak and strong singulars?
In the PIE ablaut system, different forms of nouns seem to have different stems - basically the vowels get deleted from some forms of the noun, or change to a different vowel. Eg "thought" in the accusative singular is *méntim (the -m is the accusative bit, and the 'menti' is 'thought'); but the genitive singular is *mn̥téys - the first vowel has been deleted.
But the river tripped on her by and by, lapping
as though her heart was brook: Why, why, why! Weh, O weh
I'se so silly to be flowing but I no canna stay!