Spelling

I just learned something fundamental about English spelling / pronunciation. And I'm shocked I was never explicitly taught it.

Rule:
A vowel that precedes two identical consonants is always a short vowel.

Egging
Logging
Stuff
Spill
Conning (ex vs coning)
Purr

Related rule:
Short vowels can also precede single consonants. So the former rule isn't actually super-useful.