Diagnosis
Constant jams at the rear feed (the main paper tray) are usually caused by worn or dirty pickup rollers. These rubber rollers get smooth and glossy over time and lose their grip, causing misfeeds and jams.
How to Fix It
Open the rear paper feed tray. Remove all paper. You'll see a set of black rubber rollers inside. Clean them with a lint-free cloth dampened with a little water. For better grip, you can try using a rubber rejuvenator product or, as a temporary fix, lightly roughen the surface with a clean pencil eraser. Also, make sure you're using paper that isn't damp, curled, or outside the recommended weight (16-28 lb bond is safe).
If That Didn't Work
The pickup rollers or the separation pad might be worn out and need replacement. These are consumable parts. You can order a replacement pickup roller kit (Brother part number varies) and install it yourself with some careful disassembly following a service manual or video guide.
Heads Up
When clearing a jam, always pull the paper out in the direction of paper travel (the way it was going), never straight up or backwards, to avoid tearing pieces off inside the paper path.