however, but
但 marks a contrast or exception: however, but. It signals that what follows qualifies or limits what came before.
但 combines 亻 (person) with 旦, which likely contributes the sound. The character originally expressed the idea of 'only' or 'but', though the exact historical development is uncertain.
A person 亻 standing beside the dawn 旦, as if pausing to say 'however' before the day begins.
For タン, imagine a person tanning on the beach, but then a cloud covers the sun: tan -> タン, however the tan is interrupted.