why is there a hard jerk while pulling off from stopping?

If your car has an automatic transmission, start by checking the fluid level. If it is even just a quart low it can cause that sudden jerk when trying to pull away from a stop. Read your owners book to see what type of fluid to use, or better yet stop by a trusted repair facility and ask them to see if it is low and if there are any leaks.

agree with #1 or poss trans mounts