已 is more literary and formal than 已经. In spoken Chinese, 已经 is far more common for 'already.' 已 appears mainly in written Chinese and set phrases.
grammar
Common in compound expressions: 已知 (already known), 已故 (deceased), 已久 (for a long time).
Components
ideograph
已yǐ
already; to cease
Abstract grammatical marker best learned as the perfective 'already' of 已经. Part of a tight visual triplet — 己 (self), 已 (already), 巳 (snake branch) — distinguished only by how far the left vertical breaks the upper bar. Filed under Kangxi #49 己; the radical-shape is the whole glyph, not a separable piece.
Filed under radical 己 (jǐ, #49) by convention. 己 is not a separate component in 已, so no strokes are highlighted.
