If your vehicle is showing hard to push shifter into gear, here are the most likely causes and the parts you'll need to fix each one.
The bushings connecting the shift lever to the transmission degrade over time, causing binding or misalignment that makes it physically difficult to move the shifter into the correct gates.
Old or low transmission fluid fails to properly lubricate internal components. Using the wrong fluid viscosity (a common DIY error) can also prevent manual transmission synchronizers from working correctly, resulting in stiff, notchy shifting.
If the clutch master cylinder, slave cylinder, or cable fails (or if air is left in the lines after a repair), the clutch won't fully disengage the engine from the transmission. This makes it very hard to force the shifter into gear while the engine is running.
A worn pressure plate, damaged friction disc, or failing release bearing can prevent the clutch from fully releasing, causing gear clash and requiring heavy force to shift.
In automatic vehicles, a failing brake light switch won't signal the shift interlock solenoid to release, making it extremely hard or impossible to pull the shifter out of Park.
Worn internal synchronizer rings or bent shift forks prevent the gears from matching speeds and meshing smoothly, requiring excessive force to shift. This is usually the result of long-term wear or abuse.