Formal:
* สวัสดี (sa-wat-dee) - This is the most common and formal way to say hello. It can be used in any situation where you want to be polite.
Informal:
* สวัสดีครับ (sa-wat-dee krap) - This is the informal way to say hello to a man.
* สวัสดีค่ะ (sa-wat-dee kha) - This is the informal way to say hello to a woman.
Other greetings:
* อรุณสวัสดิ์ (ah-roon sa-wat-dee) - This means "Good morning" and is used from sunrise to noon.
* สวัสดีตอนเช้า (sa-wat-dee dton chao) - This also means "Good morning" and can be used until about 12 pm.
* สวัสดีตอนบ่าย (sa-wat-dee dton bai) - This means "Good afternoon" and is used between noon and 6 pm.
* สวัสดีตอนเย็น (sa-wat-dee dton yen) - This means "Good evening" and is used after 6 pm.
Note:
* When greeting someone in Thai, it is polite to use "krap" (for men) or "kha" (for women) at the end of the greeting.
* It is also polite to smile and make eye contact when greeting someone in Thai.
Hope this helps!