Student or Learner
in the sentence the trees and the air, am I correct in pronouncing the e in the first the (the trees) as e in per and the second e (the air) as e in ear? if so what is the rule that distinguishes between the pronounciation of the two the?
Yes. The first the is in front of "T", so shortens in sound, while the second is in front of the vowel sound of "ai" so lengthens to accommodate it [though you will also hear a shortened the in front of air].