"Please be seated"，这是一种正式的说法，常用于公开场合，如会议或演讲。
例句：
"Ladies and gentlemen, please be seated as we begin the presentation."
"Please be seated while we wait for the guest of honor to arrive."
"Have a seat"，这是一种更简单的说法，常用于日常对话中。
例句：
"Have a seat and make yourself comfortable."
"Please have a seat while we wait for the others to arrive."
"Sit down"，这是一种直接的说法，常用于紧急或紧张的情况。
例句：
"Everyone, sit down and stay calm."
"Sit down before you fall down."