A few tips about long vowels

A former student of mine asked me to publish the rules for shortening vowels. Here they go:
*Only long vowels, i.e. long monophthongs and diphthongs, can be shortened.
*They can be shortened when they are 1) in unstressed position, 2) followed by an unstressed syllable in the foot or 3)when the syllable in which they appear closes in a fortis.

And one more rule (phonotactics) about long vowels: They never occur before the /ŋ/