This summary is not the rule. The rule text controls.
A witness can normally give an opinion during testimony, but there's a limit. If that opinion or inference goes straight to the exact question the judge or jury needs to decide, it cannot be allowed in as evidence. This is true even if the opinion would otherwise be an acceptable type of testimony.
In simple terms, a witness cannot tell the jury how to rule on the main issue in the case. For example, in a case about whether someone acted negligently, a witness could describe what happened or explain technical details, but could not simply say "the defendant was negligent." That final call belongs to the jury or judge, not the witness.
This rule protects the jury's role as the decision maker. It keeps witnesses, even expert witnesses, from replacing the jury's judgment with their own conclusion about how the case should turn out.
