Negation with ไม่

One word, in front

mai (ไม่) is the negative, and it goes immediately before the word it denies. mai pai, not going. mai aroi, not delicious. mai chop, do not like. There is no do-support to worry about and no agreement. Whatever the sentence, you find the verb or the adjective and put mai in front of it. Be careful of the tone: negative mai is falling, question mai (ไหม) is rising, and mai (ใหม่) with a low tone means new. Three common words, one syllable, distinguished only by tone - this is the pair of examples every teacher uses to explain why tones matter.

The negatives you will say most

mai mii (ไม่มี) is 'there is not' or 'I do not have', and it is the answer to half the questions a traveller asks. mai ao (ไม่เอา) is 'I do not want it', the polite refusal. mai pen rai (ไม่เป็นไร) is the national catchphrase: never mind, it is nothing, no problem. mai ru (ไม่รู้) is 'I do not know', with mai sap (ไม่ทราบ) as its politer form for strangers. Each of these is worth learning as a single chunk rather than assembled from parts. You will use them dozens of times a day.

Not yet, and no longer

yang mai (ยังไม่) is 'not yet', and it is much softer than a flat no. yang mai kin means I have not eaten yet, which implies I am going to. mai ... laeo (ไม่...แล้ว) is 'not any more'. mai pai laeo means I am not going after all, or I do not go there any more. laeo at the end marks the change of state. Between them these two cover most of what English does with 'still', 'yet' and 'any more', and both keep mai in its usual place in front of the verb.

Cannot, and must not

dai (ได้) after a verb means can. Negate it by putting mai in front of dai, at the end: pai mai dai, cannot go. Note the position - the negative attaches to the ability word, not to the main verb. phut thai mai dai is 'I cannot speak Thai', and phut thai mai kheng is 'I do not speak Thai well'. The second is more modest and much more useful. For prohibition, ham (ห้าม) at the front of a verb means do not, and you will see it on every sign in the country: ham suup buri, no smoking.

Saying no, gracefully

Refusing is a social act, and Thai leans on softness. Notice how many of these pair the negative with a particle, a thank you, or a hedge.

Putting mai after the verb

The negative always precedes what it negates. pai mai is not 'not going' - it is 'going?' with the question particle, which is a rather different thing to say. When you catch yourself about to negate, pause and put the mai first. The only case where the negative sits later is mai dai, where it attaches to the ability word rather than the main verb.

Being too blunt

A bare mai as an answer is grammatical and lands hard. Thai social language avoids flat refusals: soften with the particle, with mai khoi (not really), with a thank you, or with yang (not yet). The smoothest all-purpose refusal is mai ao khrap or mai ao kha with a smile. Nobody will press you further.