one, single, simple
單 centers on the idea of oneness and simplicity: a single unit, something uncomplicated, or the state of being alone rather than multiple.
單 is the traditional form of 単. It likely originated as a pictograph of a simple hunting or trapping device, later extended to mean single or simple. The modern form 単 simplifies the upper part.
The top 吅 looks like two mouths, and the bottom 十 over 田 suggests a field marked with a cross. Imagine a single, simple field with only one marker, not a complex network.
For タン, picture a single tan-colored stone in the simple field: tan -> タン, and the stone is the one and only marker.