The Swift language in depth, from the official language guide. Optionals and the unwrapping toolkit (if let, guard let, chaining — and why force-unwrap crashes), value versus reference semantics (structs vs classes, copy-on-write), properties and observers, closures and capture semantics (escaping, capture lists and retain cycles), protocols and protocol-oriented design, generics and associated types, enums with associated values, error handling (throws, try/try?/try!, Result), memory management (ARC, weak vs unowned), and modern concurrency (async/await, actors, tasks). Heavy on predict-the-output and spot-the-bug items so understanding is tested, not vocabulary. Every explanation names the principle, the trap, and a rule you code by. Authored from the official Swift documentation at its current stable release.