Hard to change gears in my v6 hyundia - 2003 Hyundai Tucson

First thing to check is oil-level in the gearbox. If far below the filler hole - refill and look for oil leaks around seals and in clutch housing.

If oil level is ok - it might need changing or when serviced recently, it might have been filled with the wrong oil.

Somewhere on the side of the gearbox there's a bolt with a seal ring. Usually this is the filler hole. Oil level check is simple, stick a finger in downwards. Should feel the oil level.

Gear boxes need the right oil viscosity and are quite sensitive - meaning changing gears gets difficult specially when cold.