該

該 is a phono-semantic compound composed of:

言 (말씀 언) — semantic component indicating relation to speech or expression.

亥 (돼지 해) — phonetic component providing the sound gāi / hae and connoting “enclosure” or “completeness.”

Originally, 該 referred to speech or statement that properly corresponds to its subject, i.e., “fitting, suitable.”

From this semantic root, it extended to the logical sense of “what should be included or is appropriate.”

應該 (응당 / 마땅히) — should, ought to

該當 (해당) — to correspond to, to be applicable

該項 (해항) — the said item or clause

該人 (해인) — the person in question

該國 (해국) — the country concerned

The earliest sense of 該 involved speech corresponding to the matter at hand — that is, “appropriate words.”

From this arose the abstract meanings “suitable,” “proper,” “due,” and finally the modal sense “should.”

「言該其理。」

“One’s words ought to accord with reason.”

Thus, 該 bridges both ethical propriety and grammatical modality — “what one ought to say or do.”

Grammatical function in Modern Chinese:

In modern usage, 該 most often appears in the compound 應該 (yīnggāi), meaning “should / ought to.”

Example:

你應該努力學習。

You should study hard.

While rarely used alone today, 該 by itself can still mean “the said” or “aforementioned” in formal writing:

該文件 — the said document.

Regional and linguistic usage:

In Classical Chinese, 該 often meant “to include, to comprise,” close to the sense of “covering all relevant matters.”

In Korean usage, 該 survives mainly in compounds like 해당(該當) - "applicable".

In Japanese, it appears in Sino-Japanese words such as 該当 (がいとう, gaitō) — “corresponding to.”

Comparison:

該 — to include; ought to; (emphasizes propriety or inclusion)

應 — should; to respond; (focuses on obligation or response)

當 — proper; appropriate; (moral and contextual correctness)

合 — to fit; to combine; (stresses harmony or fitting together)