TSUMUGU · TOCFL 1 · 字
快
快 ＝ 忄 ＋ 夬
ㄎㄨㄞˋ · kuài
adj — fast; glad
字源 FORM — what the parts do
忄義the heart
夬聲guài — gives the sound
{g,k,h}Iguài → kuài — one step inside {g,k,h}
The heart (忄) lifted — 說文's gloss is gladness. 夬 guài gives the sound, g stepped to k inside {g,k,h}. Speed grew out of the lift: 快樂 keeps the joy, 快車、趕快 run with the new sense.
故事 STORY — a scene to remember it by
信一拆開，心（忄）先亮了，回家的腳步跟著快起來。
The letter opens; the heart (忄) lifts first, and the walk home quickens behind it.
字源記憶法
意 · Meanings
(本義)喜悅、痛快— glad — the lifted heart
→快樂、愉快— happy常用
→快、趕快— fast; hurry常用
(本義) original meaning · → ⇒ each arrow = one more step of extension