History of
Darth Yoda

Here is my theory on Darth Yoda:

In Star Wars- Epidode III, we will find out that Yoda turns to the dark side.  He becomes Darth Yoda after being seduced by Darth Sidious.  Darth Yoda isn't the most fearsome fighter the Sith have seen.  In fact, far from.  He is shunned by Emporer Palpatine (who choses Anakin over Yoda), and Darth Yoda goes to Dagaboh to hide.  And all during Star Wars- Episode IV, Darth Yoda is slowly turning back to the light side, after Mace Windu's spirit visits him several times.   That is why we never see Yoda during Episode IV.  It will ruin the movie if we see an evil Darth Yoda train Luke.  We won't find out that Yoda became Darth Yoda until Episode III.