spark plug problem
 

So, I went to change my plugs out after I thought I fouled one. Replaced the back plug fine. Got to the front and there was no plug to replace. Tried to put a new one in and it wouldn't catch thread at all. Used the old one from the back and it went right in..... Please make me understand this and how can I fix the problem....

Put the two plugs next to each other, sparky end together, and line the threads up to each other... If the match and go smoothly into each groove, you have an issue... If not, go buy the correct plug...