On the surface, Thai grammar is much simpler than European languages - there's no verb inflection and tenses can often be shown just using the words "will" or "already", single and plural nouns are by-and-large the same, no noun genders, subject and object personal pronouns are the same, and any words that aren't essential to the meaning of a sentence are usually optional and can be omitted. However, it's not all as straightforward as it might appear and the points below highlight the main differences with links to each of them in more detail where needed.
Pronunciation
Thai is a tonal language, meaning each syllable or word can potentially have a different meaning depending on what tone it is pronounced with. While Thais do a good job at understanding foreigners with less-than-perfect tonal pronunciations, there are also other pronunciation difficulties that need to be tackled.
Alphabet
Thai words and sentences are written in their own unique alphabet, and sentences are written without spaces between the words as an added challenge. Though the Thai alphabet takes some work to master, if you try to learn it you'll probably discover it's not as hard as you thought.
Word Order
Unlike the esoteric word order of some other Asian languages like Japanese, the basic word order in Thai is simply subject + verb (+ object) as in English.
I live in Bangkok
ผมอยู่กรุงเทพ pŏm yòo grung tâyp
Literally: : I - live - Bangkok
Bangkok is the capital of Thailand
กรุงเทพเป็นเมืองหลวงของประเทศไทย grung tâyp bpen meuang lŭang kŏng bprà-tâyt tai
Literally: Bangkok - is - capital city - of - Thailand
However, from there the grammar starts to diverge from English fairly quickly. One common difference is that nouns at the the start of sentences are commonly followed by their pronoun:
The price is expensive
ราคามันแพง raa-kaa man paeng
Literally: price - it - expensive
The Thai language is not so similar to English.
ภาษาไทยมันไม่ค่อยเหมือนภาษาอังกฤษเท่าไหร่ paa-săa tai man mâi kôi mĕuan paa-săa ang-grìt tâo-răi
Literally: thai language - it - not - so - same - english language - however much
This can potentially cause confusion, as putting a pronoun after a noun is also the way to show possession of the noun. A related structure is to put the "topic" of the sentence first, and then use the rest of the sentence to describe it. Where the topic finishes and the sentence starts is often marked by including the relevant personal pronoun and/or with the word ก็ gôr before the next verb, but this isn't always the case.
This is particularly common where we'd say "There is/are..." instead in English:
There are many foreigners living in Thailand
คนต่างชาติที่ย้ายมาอยู่เมืองไทยมันก็มีเยอะ kon dtàang châat têe yáai maa yòo meuang tai man gôr mee yúh
Literally: foreigner - that - move - come - live - thailand - it - ก็ - have - many
But there aren't so many who can speak in Thai
แต่คนที่พูดไทยได้มันก็ไม่ค่อยเยอะเท่าไหร่ dtàe kon têe pôot tai dâai man gôr mâi kôi yúh tâo rài
Literally: But - person - that - speak - thai - can - it - not - really - many - however much
Word Omission
Thai is a strongly "pro-drop" language , and words that would be considered essential in an English sentence are frequently dropped in Thai. Both the subject and/or the object of a sentence are likely to be dropped if they can be worked out from the context, and possessive pronouns ("my", "your" etc) are usually only included in a Thai sentence if their omission would cause confusion. This has the effect of making many Thai sentences somewhat ambiguous in their meaning, and so it's commonly necessary to rely on the context to understand the intended meaning.
The pro-drop nature of Thai in addition to the "topicalization" above means that sometimes sentences that seem to be subject - verb - object in reality are object - subject - verb . For instance, the unambiguous sentence:
I've been to Phuket once before.
ผมเคยไปภูเก็ตมาแล้วครั้งหนึ่ง pŏm koie bpai poo-gèt maa láew kráng nèung
Literally: i - ever - go - phuket - มา - already - time - one
may in common speech be rendered more like:
I've been to Phuket once before.
ภูเก็ต (ผม) (ก็) เคยไปมาแล้วครั้งหนึ่ง poo-gèt pŏm koie bpai maa láew kráng nèung
Literally: phuket - (I) - (ก็) - ever - go - มา - already - time - one
To avoid confusion, at least one of the optional words in parentheses would normally be included to show where the topic ends and subject begins.
Verbs
Verbs are the cornerstone of the Thai language, and sentences with 3 or more consecutive verbs are commonplace. This is achieved by often using resultative and directional verbs where adverbs or prepositions would be used in English, using stative verbs instead of adjectives and dropping nouns and pronouns where the context makes them clear. Thai verbs don't change their form for tense, instead tenses are indicated mostly by context or with the use of words like จะ jà "will" and แล้ว láew "already".
I want to hurry back and sleep.
ฉันอยากรีบกลับไปนอน chăn yàak rêep glàp bpai non
Literally: i - want - hurry - return - go - sleep
It's not unusual for Thai sentences to consist almost entirely of just verbs, like this one with five consecutive verbs.
Adjectives
Adjectives come after the noun in Thai, rather than before it as in English. The biggest difference between Thai and English adjectives though is that Thai adjectives are also verbs (for instance, the Thai adjective for "tall" is also the verb "to be tall"), and so do not need an additional "to be" verb before them.
Nouns
Nouns in Thai have the same plural form as singular, but have the difficulty that any time a noun is quantified it must be used with its classifier. Although "classifier" may be an unfamiliar term for English speakers, it's not a completely alien concept as there is an equivalent in the way English uses "measure words" for nouns that can't be directly counted ("mass nouns"). These are nouns like "paint" or "water", and to quantify them an extra measure word is needed - "two tins of paint" not "two paints" and "several buckets of water" for instance. The measure word here works in exactly the same way as a Thai classifier, but the key difference is that in Thai even directly countable nouns like "house" or "car" also need a classifier when being quantified. The classifier for a house, for instance, is หลัง lăng and so the direct translation for "many houses" in Thai is "houses - many - lăng".
Although remembering a classifier for every single noun would seem to present a fairly significant difficulty for the learner of Thai, in reality learning 20-30 classifiers is sufficient to quantify the vast majority of nouns.
Personal Pronouns, Names and Nicknames
Although pronouns are often omitted in Thai (as above), the personal pronoun system in Thai is actually very complex and varies depending on who you're talking to, what your relationship to them is and your relative ages. Family relationship terms are often used as pronouns in Thai, so it's not uncommon to address even a complete strangers as if they were a relative. In addition, Thai women frequently use their own nickname instead of a first-person pronoun, and so refer to themselves in the third person.
Questions
The question mark isn't used in Thai; it's rough equivalent is the word ไหม măi or one of the other question particles. When answering a question, Thais don't typically use "yes" and "no" as in English, but instead repeat the main verb either by itself (for an affirmative answer) or with ไม่ mâi ("no; not") before it .
Do you like to eat Thai food ?
คุณชอบกินอาหารไทยไหม kun chôp gin aa hăan tai măi
Literally: you - like - eat - food - thai -măi
Yes, I do
ชอบ chôp
Literally: like
No, I don't
ไม่ชอบ mâi chôp
Literally: not - like
Do you have any rooms free ?
คุณมีห้องว่างไหม kun mee hông wâang măi
Literally: you - have - room - free - măi
Yes, we do
มี mee
Literally: have
No, we don't
ไม่มี mâi mee
Literally: not - have
Particles
Particles are short words added on to end of a clause or sentence, which usually can't be directly translated as such but function to change the feeling of a sentence. A single particle can make a sentence sound softer, more polite, more commanding, more sarcastic etc..., where as in English you'd usually have to re-phrase the sentence to get the same effect. Perhaps the most important ones to know are ครับ (for men) and ค่ะ/คะ (for women), which are the standard particles used to make a sentence sound polite. There are many more though, and correct use of particles is essential to speaking natural sounding Thai.