@Vedic rule

The /pANinIya has two types of rules: normal rules and vedic rules.

The purpose of the normal rules is to teach the best possible way of speaking.

The purpose of the vedic rules is to prevent the student of the vedas from panicking when they hear in the /veda anything that breaks the normal rules.

By the time grammarians started to make rules to teach the best possible way of speaking, the language of the best speakers had been drifting from centuries away from the language of the vedas. Many things in the vedas sounded like "what hath God wrought?" to the people of the time. So the teachers had to teach their students, like, "wrought is an old word for made".

Now, if you make a grammar that teaches correct language, and the grammar says "the past of make is made", then students might conclude that the "wrought" in the veda is incorrect language and therefore a misremembered word. Worse, they might figure out that as the word "wrought" occurs in the veda then it must be correct language and start saying things like "I just wrought a cake".

To avoid those eventualities, the /pANinIya is full of rules that say, like, "wrought is fine in the vedas but not nowadays".

Now a Sanskrit example. The agnisUktam अग्निसूक्तम् sings —

devo devebhir A gamat||
देवो देवेभिरा गमत्॥|"may that god come with the gods"

Which, if you are used to speaking common Sanskrit, that **devebhis देवेभिस् sounds like a mistake — it should be " **devais देवैस्". So, there are some rules somewhere that say that in the vedas both devais देवैस् and devebhis देवेभिस् are fine, while when speaking, only **devais देवैस् is kosher.