Apply foundational design principles: affordances, signifiers, constraints, feedback, and conceptual models. Use when the user mentions "why is this confusing", "affordance", "error prevention", "discoverability", "human-centered design", "fault tolerance", "mental model", "mapping", or "seven stages of action". Also trigger when diagnosing why users make mistakes, reducing product complexity, or improving error messages and feedback systems. Covers the gulfs of execution and evaluation. For usability scoring, see ux-heuristics. For iOS-specific patterns, see ios-hig-design.