Thai language is derived principally from Mon, Khmer, Chinese, Pali, Sanskrit, and, increasingly, English. Since there are no verb conjugations, verb tense indicators are easily learned, or you can even stick with the present tense. The writing system is derived from Mon and Khmer -- which in turn is from a southern Indian model -- and is composed of 44 consonants (with only 21 distinct sounds) and 32 vowels (with 48 simple and diphthong variables). It reads from left to right, often without breaks between words -- thus making some very long words! Casual visitors can get along by simply picking up simple greetings or a few polite phrases to show respect to their hosts.
Unfortunately, there is no universal transliteration system, so you will see the usual Thai greeting written as sawatdee, sawaddi, sawasdee, sawusdi, and so on. Do not be afraid of getting lost in the different spellings. Derivations of most city names are close enough for anyone to figure out. The model most often used is more similar to French than English: th usually represents our t (as in Thailand); t represents our d; ph represents our p; p sounds more like our b; kh represents our k; k sounds like g; r often sounds like l, and l can become an n. This is because Thai pronunciation is lackadaisical. Taxi drivers, in particular, often do not come from Bangkok and speak with regional accents or use dialect. Sometimes r is used merely to lengthen a vowel sound (Udon is often written Udorn), and l or r at the end of a word is pronounced more like n. The word Oriental is universally pronounced Orienten and Ubon is often written Ubol. There is no v sound in Thai, and when you see it written, as in Sukhumvit, it should actually sound like our w. There is also an ng, which sounds like letters in our word sing, used as an initial consonant and difficult for English speakers to hear and pronounce though the distinction can be important: noo means rat or kid (informal for child), but ngoo means snake.
Central Thai is the official written and spoken language of the country, and most Thais understand it, but there are three other major dialects: Northeastern Thai, spoken in Isan, and closely related to Lao; Northern Thai, spoken in the northwest, from Tak Province to the Burmese border; and Southern Thai, spoken from Chumphon Province south to the Malaysian border. Each of these dialects also has several variations. The hill-tribes in the North have their own distinct languages, closely related to Burmese or Tibetan.Just as in English, there are various degrees of formality, and words that are acceptable in certain contexts are impolite in others. The most common word for eat is khin (also written gin), khin khao means "eat rice" (but is used to indicate or inquire about eating a meal in general); thaan is more polite, while raprathaan is reserved for royalty.
