In a legal malpractice claim, the plaintiff must prove the following:
"Prove" means persuading the decision-maker (jury or judge) on a more-likely-than-not basis (otherwise known as a preponderance of the evidence standard of proof).
Legal malpractice lawsuits often involve two distinct sets of issues and evidence: Liability and damages. Liability refers to whether the attorney was negligent. Damages refers to the harm (usually economic) that the client suffered as a result of any negligence.
In legal malpractice cases, proving damages when the mishandled matter was a claim or lawsuit usually involves persuading a jury or judge that, more likely than not, the plaintiff would have prevailed in the underlying lawsuit if the first lawyer did not make negligent mistakes. To do this, the plaintiff must present the underlying case ("case-within-a-case") in the malpractice trial so the jury can determine if the plaintiff would have prevailed but for the lawyer’s negligence. Otherwise, it cannot be said that the lawyer’s mistake harmed the client.
From this perspective, legal malpractice cases are often essentially two cases in one. One "case" deals with the lawyer’s representation of the client and alleged mistakes, and the other is the underlying case or transaction that the defendant lawyer handled. The underlying case might not have been presented at trial due to the first lawyer’s mistakes, or it might have been tried with mistakes and a bad result. If the former, then the malpractice trial will be the first time the case-within-a-case is presented to a court or jury. If the latter, then the malpractice trial will involve a re-examination of the first trial, and often will feature the presentation of different or additional evidence and/or arguments that should have been presented in the first trial but were not.
In a legal malpractice case that involves something other than a mishandled claim or lawsuit, including for instance poor advice or document drafting relating to a business deal, real estate transaction, divorce property settlement, or estate planning, the damages question may involve a different set of "what-ifs." The concept is the same, however – a direct comparison between the actual economic/financial circumstances of the client, and the hypothetical circumstances assuming the matter had been handled properly.
In some legal malpractice cases, the defendant disputes that negligent mistakes were committed and also asserts that, even if the acts and omissions claimed by the plaintiff were below the standard of care, they did not cause any damage to the client. Such a case is said to present issues of liability and damages. Often, however, in legal malpractice cases the defendant either admits negligent mistakes, or cannot genuinely contest them, or the question of negligence is simple and straightforward. In such cases, the malpractice claim is largely or exclusively about damages. In other legal malpractice cases, the question of damages may not be the subject of any genuine dispute, and may even be admitted or stipulated, and the entire case revolves around whether the lawyer’s did or failed to do anything that was below the standard of care.