Why does Thai have no words for "yes" or "no"?

Question

There are words like /chaj/ and /mej/, but as far as I understand they are not exactly yes and no.

Answer 1

Short answer: Because Thai language has other tools for expressing polarity (affirmation and negation).

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Polarity is a grammatical category for expressing the speaker's assertion that a certain clause is true (positive polarity, or affirmation) or false (negative polarity, or negation).

There are numerous linguistic tools to encode both polarities, and each language chooses its own set of these tools.

English has much more than simple "yes" and "no". Consider yea and nay, the two forms originally intended for answering questions "is it...?" (yes and no were originally used for answering inverted questions "is it not...?"

English also has an essential difference between no (negative), no (qualifier), and not.

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As I said, Thai has different tools for expressing polarity:

First off, ไม่ [mâj] is a negative particle (NEG), quite similar to English not. Likewise in English, it is also used with verbs only:

ผมเอา [pʰǒm ʔaw] — I want¹
ผมไม่เอา [pʰǒm mâj ʔaw] — I don't want¹

¹ hereinafter I'm skipping polite particles for brevity; don't omit polite particles in real speech.

The positive polarity is encoded with verbs; and the negative polarity simply adds the NEG particle mentioned above:

• repeating the verb:

  — คุณเอาไหม [kʰun ʔaw mǎj] — "do you want…?", literally "you want…, {INTERROG}"
  — เอา [ʔaw] — {I} want
  — ไม่เอา — [mâj ʔaw] — {I} don't want

• an indirect flavor of "to be" is affirmed with ใช่ [tɕʰâj]:

  — ฟ้าใสไหม [fáː sǎj mǎj] — {the} sky {is} clear, isn't it?
  — ใช่ [tɕʰâj] — {it} is.