Well here is the "rule" I taught today. I say "rule" because in the English language there is always an exception to the rule.
The CV phonics rule (Consonant Vowel) states that a reader can make a long sound at the end of a short word if it ends in a vowel.
For example: hi, he, she, me, so, go
In hem, the vowel is short because it's stuck in between two consonants (CVC).
In the word he, the e is able to stretch out and say his long sound (his name)