Putting on and taking off pedals is often difficult. The threads on the pedals go in the direction of the pedal. If you don’t grease the threads and take them off once in a while they practically fuse themselves to the crank arms. I found this out at Bike Virginia. The transport crew was loading our bikes onto a truck bound for the start of the ride in Lexington, Virginia. They tried and tried and tried to remove the pedals from my trusty old Trek 1200. Finally, they gave up.
They were probably using a pedal wrench with an opening at 3 and 6 o’clock or at 6 and 12 o’clock. When you look at these wrenches you think, “Works for me.” Well, no. If your pedals are stuck, these wrenches will only succeed in knuckle scrapes and frustration. I know. I have three different kinds of these wrenches. They suck.
Instead you need a wrench that has an opening at 10 or 2 o’clock. For some mysterious reason this gives you just the right amount of mojo to loosen even the tightest pedal. After years of hassle, I finally coughed up the dinero and bought a Park Tool PW-4 wrench. It has a long, beefy handle for leverage and the offset openings are trademarked by Hogwarts because they can do magic. You just put it on the pedal and pop! the pedal threads release. No muss. No fuss.
Park Tool sells them for $35.
Worth it.
A Few Spokes Shy of a Wheel 
When I was moving back to Washington from Germany, I attempted to take my bike apart by myself to get it packed to ship home. First thing I was going to take off were the pedals…and it was an instant fail. I had to have two men help me, and they struggled too. I now have a better understanding of pedals and have assembled many bikes in my time at REI, but man I will never forget that!
Part of the trick is to not put them on super tight in the first place but this wrench is great