These languages are written with slightly different scripts, but are linguistically similar and effectively form a dialect continuum.

[11][12] Ethnic minorities today are predominantly bilingual, speaking Thai alongside their native language or dialect.

[13] The Thai writing system has an eight-century history and many of these changes, especially in consonants and tones, are evidenced in the modern orthography.

According to a Chinese source, during the Ming dynasty, Yingya Shenglan (1405–1433), Ma Huan reported on the language of the Xiānluó (暹羅) or Ayutthaya Kingdom,[e] saying that it somewhat resembled the local patois as pronounced in Guangdong[14]: 107  Ayutthaya, the old capital of Thailand from 1351 - 1767 A.D., was from the beginning a bilingual society, speaking Thai and Khmer.

Bilingualism must have been strengthened and maintained for some time by the great number of Khmer-speaking captives the Thais took from Angkor Thom after their victories in 1369, 1388 and 1431.

An investigation of the Ayutthaya Rajasap reveals that three languages, Thai, Khmer and Khmero-Indic were at work closely both in formulaic expressions and in normal discourse.

The maximal four-way occurred in labials (/p pʰ b ʔb/) and denti-alveolars (/t tʰ d ʔd/); the three-way distinction among velars (/k kʰ ɡ/) and palatals (/tɕ tɕʰ dʑ/), with the glottalized member of each set apparently missing.

The major change between old and modern Thai was due to voicing distinction losses and the concomitant tone split.

The above consonant mergers and tone splits account for the complex relationship between spelling and sound in modern Thai.

[g] หม ม หน น, ณ หญ ญ หง ง ป ผ พ, ภ บ ฏ, ต ฐ, ถ ท, ธ ฎ, ด จ ฉ ช ก ข ค, ฆ อ ฝ ฟ ศ, ษ, ส ซ ฃ ฅ ห หร ร หว ว หล ล หย ย อย Early Old Thai also apparently had velar fricatives /x ɣ/ as distinct phonemes.

Pittayaporn (2009[full citation needed]), for example, reconstructs a similar system for Proto-Southwestern-Tai, but believes that there was also a mid back unrounded vowel /ə/ (which he describes as /ɤ/), occurring only before final velar /k ŋ/.

Standard Thai distinguishes three voice-onset times among plosive and affricate consonants: Where English makes a distinction between voiced /b/ and unvoiced aspirated /pʰ/, Thai distinguishes a third sound – the unvoiced, unaspirated /p/ that occurs in English only as an allophone of /pʰ/, for example after an /s/ as in the sound of the p in "spin".

Of the consonant letters, excluding the disused ฃ and ฅ, six (ฉ ผ ฝ ห อ ฮ) cannot be used as a final and the other 36 are grouped as following.

Thai has specific phonotactical patterns that describe its syllable structure, including tautosyllabic consonant clusters, and vowel sequences.

In core Thai words (i.e. excluding loanwords), only clusters of two consonants occur, of which there are 11 combinations: The number of clusters increases in loanwords such as /tʰr/ (ทร) in อินทรา (/ʔīn.tʰrāː/, from Sanskrit indrā) or /fr/ (ฟร) in ฟรี (/frīː/, from English free); however, these usually only occur in initial position, with either /r/, /l/, or /w/ as the second consonant sound and not more than two sounds at a time.

The language being analytic and case-less, the relationship between subject, direct and indirect object is conveyed through word order and auxiliary verbs.

'In order to convey tense, aspect and mood (TAM), the Thai verbal system employs auxiliaries and verb serialization.

'ฉันchan/tɕʰǎnจะchatɕàʔกินkinkīnที่thithîːนั่นnannânพรุ่งนี้phrungnipʰrûŋ.níː/ฉัน จะ กิน ที่ นั่น พรุ่งนี้chan cha kin thi nan phrungni/tɕʰǎn tɕàʔ kīn thîː nân pʰrûŋ.níː/'I'll eat there tomorrow.

[26] The imperfective aspect marker กำลัง (kamlang, /kām lāŋ/, currently) is used before the verb to denote an ongoing action (similar to the -ing suffix in English).

เขาkhao/kʰǎwยังyangjāŋเขียนkhiankʰiǎnอยู่yujùː/เขา ยัง เขียน อยู่khao yang khian yu/kʰǎw jāŋ kʰiǎn jùː/He is still writing.เขาkhao/kʰǎw3SGยังyangjāŋIMPเขียนkhiankʰiǎnwriteไม่เป็นmai penmâj pēn/unable toเขา ยัง เขียน ไม่เป็นkhao yang khian {mai pen}/kʰǎw jāŋ kʰiǎn mâj pēn/3SG IMP write {unable to}He can't even write yet.The marker ได้ (dai, /dâːj/) is usually analyzed as a past tense marker when it occurs before the verb.

[26] เขาkhao/kʰǎwได้daidâːjไปpaipājเที่ยวthiaotʰîawเมืองmueangmɯ̄aŋลาวlaolāːw/เขา ได้ ไป เที่ยว เมือง ลาวkhao dai pai thiao mueang lao/kʰǎw dâːj pāj tʰîaw mɯ̄aŋ lāːw/He visited Laos.

Possession in Thai is indicated by adding the word ของ (khong) in front of the noun or pronoun, but it may often be omitted.

แก้วkaew/kɛ̂ːwglassใบbaibājCLหนึ่งneungnɯ̀ŋ/oneแก้ว ใบ หนึ่งkaew bai neung/kɛ̂ːw bāj nɯ̀ŋ/glass CL one"one glass"In the previous example khon (คน) acts as the classifier in the nominal phrase.

In which the pronoun or noun modifier can be used separately for นี่ (ni, /nîː/), while นี้ (ni, /níː) refers to be after the prepositions and classifiers for example: นี่ni/nîːthisคือkheukʰɯ̄ːisโต๊ะtotóʔtableใหม่maimàj/newนี่ คือ โต๊ะ ใหม่ni kheu to mai/nîː kʰɯ̄ː tóʔ màj/this is table new"this is a new table"วางwang/wāːŋput downถ้วยthuaitʰûajbowlบนbonbōnonนั้นnannán/thatวาง ถ้วย บน นั้นwang thuai bon nan/wāːŋ tʰûaj bōn nán/{put down} bowl on that"put a bowl on there"The syntax for demonstrative phrases, however, differ from that of cardinals and follow the pattern noun-classifier-demonstrative.

For example: The particles are often untranslatable words added to the end of a sentence to indicate respect, a request, encouragement or other moods (similar to the use of intonation in English), as well as varying the level of formality.

[36][citation needed] Rhetorical, religious, and royal Thai are taught in schools as part of the national curriculum.

Chinese-language influence was strong until the 13th century when the use of Chinese characters was abandoned, and replaced by Sanskrit and Pali scripts.

As a result, it is impossible for Thais, past and present, to engage in a conversation without incorporating Khmer loanwords in any given topic.

Since the beginning of the 20th century, however, the English language has had the greatest influence, especially for scientific, technical, international, and other modern terms.

Their influence in trade, especially weaponry, allowed them to establish a community just outside the capital and practise their faith, as well as exposing and converting the locals to Christianity.