Essential Thai words and phrases — even a few go a long way in Bangkok.
Thai is tonal — the same syllable spoken in different tones means different things; mispronunciation can cause confusion; Thais will generally understand context.
Sawasdee (hello/goodbye) + krap (male) / ka (female) as polite particle; khob khun (thank you); mai pen rai (no problem/never mind); chai (yes); mai chai (no).
Aroy mak (very delicious); pet nit noi (a little spicy); mai pet (not spicy); check bin (bill please); aroi? (delicious, asking for feedback).
Thi nai? (where?); pai... (going to...); chao lot fai fah (BTS skytrain); raakhaa thao rai? (how much?).
Adding krap (men) or ka (women) after any phrase makes it polite; wai (prayer hands greeting) shows respect; Thais are forgiving of foreigners' language attempts.
Sawasdee (hello), khob khun (thank you), and mai pen rai (no worries/it's fine) cover the vast majority of situations where a foreigner might want to say something in Thai — pronunciation matters less than the attempt itself.
Thai is tonal — the same syllable pronounced with different tones can mean entirely different things; see the Thai tones guide for why this trips up so many learners.
Adding "krap" (for men) or "ka" (for women) at the end of sentences signals politeness — even a single word like "khob khun krap" sounds noticeably more polite than "khob khun" alone.