i-adjective
Describes a surface or shape that is flat, without bumps or curves. Can also be used figuratively for something straightforward.
この道は平たくて歩きやすい。
This road is flat and easy to walk on.
平たい皿に料理を
Serve the food on a flat plate.
i-adjective
Used for language or explanations that are easy to understand, without complicated terms. Often appears in set phrases like 平たく言えば (to put it plainly).
See also: 平たく言えば
平たく言えば、それは無理だということだ。
To put it plainly, it means it's impossible.