If your car jerks and produces a loud knocking sound when you accelerate from a complete stop in first gear, there are a few potential causes that you can check on your own before seeking professional help.

1. Clutch Issues: One possibility is that your clutch may be worn out or slipping. When the clutch does not engage properly, it can cause the car to jerk and produce a knocking sound. You can check the clutch by observing the engagement point, which should be smooth and consistent. If you notice any slipping or grinding sensations, it may indicate a clutch problem.

2. Transmission Problems: Another potential cause could be related to the transmission. Issues with gears or the transmission may cause the car to jerk and produce a knocking sound. Check the transmission fluid level to ensure it is at the correct level. If it is low or dirty, it could lead to transmission problems. Additionally, check for any leaks under the car, as leaks can also cause transmission issues.

3. Engine Misfiring: An engine misfire can cause the car to jerk and produce a knocking sound. This may occur due to a faulty spark plug or ignition coil. Check the condition of the spark plugs and ignition coils, ensuring they are clean and in good working condition. If any spark plugs or coils are worn out or damaged, they will need to be replaced.

4. Engine Mounts: Worn or damaged engine mounts can also lead to jerking and noises. The engine mounts are responsible for securing the engine in place, absorbing vibrations, and minimizing movement. If the mounts are worn or broken, the engine may shift during acceleration, causing the car to jerk and produce knocking sounds.

It is important to note that diagnosing and repairing these issues requires technical expertise. If you are unable to identify the root cause or if the problems persist, it is advisable to consult a professional mechanic for a thorough inspection and proper diagnosis. They will have the necessary tools and knowledge to accurately diagnose and repair any underlying problems.