Maybe it’s a really old and rusty blade that has an extremely hard time opening.
In such a scenario, a cleaning of the blade is required. Even though cleaning a folding blade sounds like a tedious task, it’s quite a simple one, and the results are a good and usable folding knife.
Cleaning the blade
First check to see if the knife is rusty. Is it slightly orange in colour? Maybe powdery and white? That can happen if the metal in the knife has oxidized. In that case, disassemble the knife and soak it in mineral oil. Leave it like that for a day or two, then give it a good cleaning.
If your blade has wooden parts too, then soaking it in mineral oil is not a good solution. Instead, you can use scotch tape to mask the wooden areas, or use a cotton swab for precision cleaning.
Because you can’t just soak it in mineral oil, this process of cleaning must be repeated over the course of a week. Clean it at least four times in a week.
Loosening the screw
In case your blade is not rusty, but just opens hard, then the only thing necessary is to loosen up the screw.
What you need to do is inspect the base of the blade and find a screw. There is usually only one. Find the proper screwdriver, and carefully unscrew it. Make sure you don’t drop it. Instead, just loosen it slightly, and then try to open the blade? If the blade is still too hard to open, repeat the process until you reach the position you desire.
An interesting video that is explaining everything can be seen below: