To establish a claim for negligence, the plaintiff must show that the defendant had a duty of care to the plaintiff, then breached that duty, which then caused injury to the plaintiff. The basic negligence rule is that everyone must take reasonable care to avoid causing injury to others.
Human beings are always finding new ways to be negligent. In some instances, the law sets specific standards for what constitutes negligence. This is called negligence per se. Usually, though, whether a defendant has been negligent is a judgment call about whether that defendant has exercised reasonable care. The plaintiff bears the burden of proving negligence by a preponderance, or “greater weight” of the evidence.