KanjiJLPT N3
Simple

simple, one, single, merely

Mnemonic

A monk shaves his hair, sheds his armor, and owns only one plain robe. Life becomes simple when you strip away every unnecessary layer — nothing left but peaceful emptiness.
Additional thoughts
Picture a warrior-turned-monk: first the hair falls away, then heavy armor clanks to the ground, leaving him with one garment. Each removal simplifies his existence further. Top to bottom in the kanji: hair on top, armor in the middle, one stroke at the base.
Quick recall
Shave hair, shed armor, keep one robe — life is simple.

Details

The keyword for 単 is simple. This kanji conveys the idea of something being singular, uncomplicated, or standing alone without complexity. It carries the sense of plainness and straightforwardness, referring to things that are not compound or multifaceted. By extension, it can also imply something being mere or nothing more than what is presented.
On'yomi
たん
Kun'yomi
ひとえ