How and when to use 形（かたち、けい、かた）

Question

What exactly are the scenarios where かたち、けい　or かた would be used? For example, in this sentence 「 輪の形に動かす」, is it かたち or かた？

Answer 1

かたち and かた are different, though related, words.

かたち is the more common and general-use of the two, and corresponds fairly closely in meaning to the English word "shape", referring to the basic outward appearance of something, often in a 2D rather than a 3D sense. When you see 形 used on its own, it's generally safe to assume that かたち is the intended reading unless you have reason to think otherwise, and this includes your example sentence.

かた is a somewhat more technical term, and usually refers to one of a few different defined forms or types of something. In most of its uses, it's more commonly written with the kanji 型 than 形, though they can be interchangeable. It's also much more common than かたち in kun-reading compounds, particularly in its usage as a suffix meaning "shaped like X" (usually pronounced ～がた due to rendaku, eg. 菱形{ひしがた} "diamond-shaped", 卵型{たまごがた} "egg-shaped").

けい is the on-reading of the kanji, and to my knowledge it's not generally used as a word on its own. Like most on-readings, it largely occurs in Sino-Japanese compound words (kango).