Paanini created a list of 3950 rules, each of which is a sutra, nonsense phrase to be memorized.

Consider the word glass. We create the plural by adding an s, but the rule is that we insert an e before the s. For the word cat, we don't insert an e, the s is simply added to the word. For the word dog, we add an s but pronounce it like a z. That is the kind of thing the rules deal with.

Here are the rules:

http://gretil.sub.uni-goettingen.de/gretil/1_sanskr/6_sastra/1_gram/paniniiu.htm

The rules use lists of sounds. Instead of listing letters in some traditional random ways, like the alphabet, he grouped similar sounds and gave each group a name. He call this table the Shiva sutras, shown here:

https://www.learnsanskrit.org/panini/shivasutras/

The name comes at the end of the list, so the semivowels l and r are referred to as k.

Here's some idea of how the rules work:

He groups these lists sometimes by naming the first letter of the first list and the name of the last list, so aten means a, i, u, l, r, e and o. (at mean short a).

On of the rules is at-eṅ guṇaḥ which defines the word guṇaḥ as a, i, u, l, r, e and o, the short vowels. (l and r are sometimes vowels in Sankrit) If you search the word guṇaḥ, it's used 10 times. As far as I know it's a nonsense word he invented for his rulebook.