Thai (Central Tai, Siamese, Standard Thai, Thaiklang) is a member of the Southwestern branch of the Tai-Kadai language family.The word thai means ‘free’ in the Thai language. Its closest relatives are Lao, Shan, and Southern Thai. Thai is spoken as a first language by 20.2 million people and by 40 million speakers as a second language in Thailand. It is also spoken in the Midway Islands, Singapore, United Arab Emirates, and the United States (Ethnologue). 

The Thai people originally came from China. They moved into the Indochina peninsula some 2,000 years ago. Thai is first attested by an inscription dating back to the late 13th century AD. Initially, the Thais were dominated by the Mon and then later by the Khmer. They became independent by the mid 13th century AD. Their country then became known as Siam, and the language as Siamese. In 1939, Siam became the Kingdom of Thailand.

Thai is the official national language of Thailand. It is used as a medium of instruction in schools, by the media and in all government affairs. An estimated 80% of Thaland’s population speaks Thai as their first or second language. Thai serves as a lingua franca, or language of wider communication, for speakers of the country’s 74 different languages. Most newspapers and periodicals are in Standard Thai, although there is a number of them in Chinese, English, and Malay. The Thai government promotes the use of Standard Thai.

Regional dialects
There is no universal agreement about the dialect situation in Thailand. Some scholars consider only Khorat Thai as a dialect of Thai, while others distinguish between Bangkok Thai and Central Thai.

Social dialects
There are two varieties of Thai that cut across geographical regions. They differ in vocabulary and, to some extent, in grammar.

• The high form is used when talking about members of the royal family, high ranking Buddhist clergy, and persons of high social status. The high form contains many loanwords from Sanskrit and from Khmer.
• The low form is used in everyday situations and when talking about persons other than those listed above under high form.

Structure

Vowels
Thai has nine vowel phonemes, i.e., sounds that make a difference in word meaning. Vowels can be short or long. Vowel length makes a difference in word meaning. This results in 18 vowels. Long vowels have a duration which is about twice as long as short vowels. In addition, vowels form diphthongs by combining withsemivowels, e.g., /ia/.

• /ɛ/ = e in bet
• /ɨ/ = similat to e in roses
• /ə/ = a in about
• /ɔ/ = o in bog

Consonants
There are twenty consonant phonemes, i.e., sounds that make a difference in word meaning. They are listed in the table below. There are no consonant clusters, and only a few consonants can occur at the end of syllables.Thai does not have a contrast between voiceless and voiced stops such as between /p/ and /b/. Instead, there is a contrast between voiceless plain (unaspirated) and aspirated stops, e.g., p – pʰ.

• /c/ = similar to k in keen
• /ŋ/ = ng in song

Tones
Thai is a tonal language in which otherwise identical words can be distinguished by five tones. The following examples are from Strecker, D. (1990). Tai Languages. In B. Comrie (ed.). The World’s Major Languages, pp. 749-775. Oxford University Press.

• Syllables that end in a long vowel, a semi-vowel, or a nasal can have all fine tones.
• Syllables that end in a short vowel and a stop, or those that have no final consonant have a low or a high tone.
• Syllables with a long vowel followed by a stop usually have a low or a falling tone.

Stress
Linguists generally agree that stress in Thai distinguishes between otherwise identical words and that it is most prominent in final position.

Thai is an analytic language which means that it does not use inflections to represent grammatical relations, such as case, gender, number or tense.

Nouns

• Nouns are not marked for number, gender, or case.
• Thai uses a system of classifiers that follow the numeral and precede the noun. There are separate classifiers for different classes of people, objects of different shapes and functions, clothes, foods, animals, e.g., tua for animals, khone for people, ahn for objects in general.
• Possession is expressed by juxtaposion of the object possessed next to possessor, or by a particle.

Pronouns
Thai has a complex system of pronouns. The choice of pronouns in any given situation is determined by the sex, age, social position and the attitude of the speaker towards the addressee. Different pronouns are used in different situations. Below are several examples.

• First person
  Thai pronouns for ‘I’ are different for male and female speakers. In formal settings, men will use phome and women will use dee-chan. However, it is common to drop these formal pronouns in face-to-face conversations or to use kinship terms, such as aunt, uncle, younger/older sibling, or first names instead.
• Second person

• Third person

Verbs
Verbs are not inflected for person, number, tense, aspect, or mood. These functions are determined by context or by adverbs and expressions of time.

Particles
Thai has three broad classes of particles that occur at the end of sentences.

• Politeness particles 
  These are used to express deference towards the addressee. Polite language in Thai requires that a politeness marker be at the end of every phrase. These are used to express deference towards the addressee. Markers differ according to the gender of the speaker. Men will show deference by ending their questions and statements with khrahp to show respect and refinement. Women end their questions and statements with khah.
• Mood particles
  These express the attitude of the speaker towards the situation at hand, such as urging, persuading, encouraging, etc.
• Question particles
  Different question particles are used in yes-no questions, depending on whether or not the speaker has expectations as to what the answer might be.

Word order
Word order in colloquial Thai varies depending on what is known and what is new information in the sentence. This means that Subject-Verb-Object, Subject-Object-Verb, and Object-Subject-Verb word orders are all possible. In Standard Thai, however, Subject-Verb-Object word order is considered to be the norm. Modifiers follow the nouns they modify.

Historically, Thai has borrowed words from Sanskrit and Pali, particularly religious terms. Chinese has also been an important source of early loans, contributing numerals and a few hundred basic terms. In the modern era, culinary and commercial vocabulary has also entered Thai from Chinese. English has also become an important source of loans, especially in the popular cultural sphere, in the mass media, and in commerce.

The most productive derivational processes in modern Thai are compounding and reduplication. Below are some examples:

• Compounding
  rótfay ‘train’ from rót ‘car’ + fay ‘fire’
• Reduplication
  dii  ‘good,’diidii ‘rather good,’ dèk ‘child,’ dèkdèk ‘children’

Below are some basic words and phrases in Thai.

Below are Thai numerals 0-10 in romanization and in Thai script.

Writing

The Thai script, known as the Sukhothai, was developed in the mid-13th century, shortly after the Thais gained their independence. It is probably based on the Old Khmer script. The Khmer script, a descendant of the Brahmi script of India, was one of the earliest writing systems used in Southeast Asia, dating back to the 7th century AD. It is a syllabic script in which each consonant has an implicit short /a/ vowel if it stands by itself and an implicit short vowel /o/ if it is followed by another consonant. These vowels can be suppressed or replaced by other vowels that are written before, after, above, or below the consonant. Tone markers are placed above the initial consonant of a syllable or on the last consonant of an initial consonant cluster.

Take a look at Article 1 of the Universal Declaration of Human Rights in Thai script and in romanization. There is no universal standard for romanizing Thai. Rules published by the Thai Royal Institute for romanizing Thai words, are not universally applied.