Arrest Powers, Search and Seizure, and Citizen's Arrest
Two kinds of arrest authority
Officer warrantless arrest authority in the District is codified (D.C. Code § 23-581). The core categories are a felony supported by probable cause and a misdemeanor committed in the officer's presence, with limited additional situations. Always be able to name the prong you are relying on; questions frequently describe facts that fit one prong and offer an answer citing another.
A private person in DC may arrest for a felony, or for a misdemeanor involving a breach of the peace, committed in that person's presence. A tip, a hunch, or "he looked suspicious" never supports a citizen's arrest - the citizen must witness the offense. The arrestee must be delivered to law enforcement or a magistrate without unnecessary delay, and the citizen can face civil liability for an unlawful detention. Detaining someone for a past misdemeanor the citizen did not observe is a classic wrong answer choice.
Searches
Start from the warrant requirement and probable cause, then apply the exceptions: search incident to lawful arrest (narrowed for vehicles after Gant), consent (voluntary, from a party with authority, within scope), plain view (lawful vantage point plus immediately apparent incriminating character), exigency, inventory, and the automobile exception. A Terry stop needs reasonable, articulable suspicion of criminal activity; a frisk needs separate suspicion the person is armed and dangerous.
Traps and rule of thumb
Traps: treating reasonable suspicion and probable cause as interchangeable; searching incident to an offense that results only in a citation; expanding a traffic stop without new justification. For every search, state the three words: authority, scope, justification.