应 yīng is the simplified form of traditional 應. The traditional 應 carries 心 (xīn, heart) at its base: the older sense is something the heart answers to, a feeling or a duty that calls for a response. From that root grow both readings, one tilting toward what the heart ought to do and one toward how it responds. The upper portion of 應 supplies the sound.

The mainland simplification 应 keeps the 广 (guǎng, shelter) frame as its outer radical and reduces the dense interior of 應 to a compact base, dropping the explicit 心. The two forms are read identically; only the script differs.

应 is a polyphone, and the reading splits the senses cleanly:

1. yīng (level tone). Ought to, should: 应该 yīnggāi (should), 应当 yīngdāng (ought to), 理应 lǐyīng (by rights ought to). This is the modal of obligation, what one is duty-bound to do.
2. yìng (falling tone). To respond, to answer, to meet: 反应 fǎnyìng (a reaction), 答应 dāying (to agree, to promise), 适应 shìyìng (to adapt), 供应 gōngyìng (to supply), 应付 yìngfu (to cope, to handle). This is the active sense of answering a call or meeting a demand.

The thread through both readings is the same: a heart that answers. When the answer is the inner pull of duty, what one should do, it is yīng; when the answer is an outward act, a reply or a response to circumstance, it is yìng.

In the level tone yīng, 应 is the modal of obligation. The most common word is 应该 yīnggāi, the everyday "should" or "ought to." Its close cousin 应当 yīngdāng means the same thing but is a touch more formal, frequent in writing, rules, and contracts. The bare 应 yīng also appears alone in compounds such as 理应 lǐyīng (by rights ought to), where it is unmistakably the obligation sense.

Hear the level tone and think duty: what the heart is bound to answer to. The same glyph in the falling tone yìng turns from "what I should do" to "how I respond," which is the subject of the next two sections.

Let the tone fall from level yīng to falling yìng and the character turns from "what I ought to do" to "how I answer." 反应 fǎnyìng is a reaction, the way a body, mind, or system responds to a stimulus. 答应 dāying is to answer a request with a yes, to agree or promise. 响应 xiǎngyìng is to echo or respond to a call, often a public one. 回应 huíyìng is simply to reply. The common thread: something comes in, and 应 yìng is the answer that goes back out.

The falling-tone yìng widens from "to answer a call" to "to meet a situation." 适应 shìyìng is to adapt, to fit oneself to new conditions. 应付 yìngfu is to cope with or handle, sometimes with a hint of doing only the minimum to get by. 供应 gōngyìng is to supply, to meet a demand with goods. 应用 yìngyòng is to apply, to put to practical use. In each, 应 yìng is the act of rising to meet what the world presents.

Hold one idea at the center of 应: a heart that answers. In the traditional 應 the 心 sits at the base, and everything the character means is some way the heart answers a call. When the call is the inner pull of duty, the answer is what you ought to do, and the tone is level yīng: 应该 (should), 应当 (ought to).

Then let the tone fall to yìng and the answer turns outward, into the world. A stimulus arrives and you give a 反应, a reaction. A request arrives and you 答应, you agree. A new situation arrives and you 适应, you adapt. A demand arrives and you 供应, you supply. Hear level yīng and think duty, what you should do; hear falling yìng and think response, how you meet what comes. The tone is the switch between the two halves of the character.