A window that's hard to open or close is almost always a dirt and lubrication problem. Clean the tracks and apply silicone spray and most sticky windows open freely within minutes.
Run a putty knife along the window tracks to scrape out compacted dirt and old dried lubricant. Wipe the inside of the tracks thoroughly with a damp microfiber cloth. Let dry before lubricating.
Spray silicone lubricant along the full length of both window tracks. Open and close the window several times to distribute the lubricant throughout the track.
If the window is painted shut, run a utility knife along the seam between the window and the frame to break the paint seal. For swollen wood frames, allow humidity levels to drop and the window will usually free itself naturally.