If the starter drive is in the "engage" position, that is with the gear almost touching the front plate of the starter, it is much more difficult to install the starter.  Most of us have turned the starter drive by hand out of curiosity, and once it screws itself in it won't retract.  If you spin the gear with your fingers, it will turn easily one direction and make a ratcheting sound.  What I do is hold the gear against the wire wheel on my bench grinder, spinning it in the ratcheting position.  If I spin it fast enough, it will retract.  Leave it there, and try installing the starter again.  If a wire wheel isn't available, I have once done it against a jacked up spinning rear wheel.  This is a rather risky way to do it though.  If your starter gear is about 5/8-3/4 inch away from the front plate, it is already retracted and the above won't help.