The plaintiff must establish that the defendant was careless or reckless in order to establish a case for personal injury. The basic negligence rule is that everyone must take reasonable care to avoid causing injury to others. The level of care required varies according to the circumstances, including the relationship between the parties.
Human beings are always finding new ways to be negligent. In some cases the law sets specific standards for what constitutes negligence. Usually, though, whether a defendant has been negligent is a judgment call about whether that defendant has exercised reasonable care. Perhaps the most important thing to remember about negligence is that the plaintiff bears the burden of providing reasonable (though not necessarily conclusive) proof that negligence was present. That an accident occurred does not, of itself, establish that the defendant was negligent. Some accidents occur even without negligence.