You would certainly be safe to use 'chai' when the sentence ends with 'chai mai' [as that specifically seeks a yes/no answer] but krap/klap is universal for 'I agree/yes/yup/okay/I get it/uh-huh'...heck, properly/politely you're probably even hear 'chai klap'--krap pom is even better when talking to police or immigration The "Help" section of Talking Thai doesn't really distinguish between chai/krap but it does offer polite/formal/slang/verboten versions of common words and that really comes in handy. The Help section is really a grammar/spelling/alphabet guide-tutorial and it links directly from each word if desired. Even covers tougher things like interpreting silent letters...