The full-height hook that pierces the whole character is the 亅 radical and is what the Xinhua dictionary indexes 事 under. Visually it represents the long staff or brush-handle held in the clerk's grip, running from cap to base.
Top horizontal caps the structure and historically represented the banner or insignia an official held. It sits across the top of 口 below and is part of the etymological 史/吏 (record-keeper) family from which 事 descended.
Small enclosed box is 口 mouth; in 事 it stood for an inkpot or vessel attached to the banner the official held. Together with the top stroke it builds the upper half of the bureaucratic-scribe image at the core of this character.
Three-stroke 彐 here is the contracted shape of a hand gripping the banner-staff. Combined with the central vertical-hook below, it completes the image of a clerk holding up the official record being kept.
