Please have a seat.为什么要用a

Please have a seat.为什么要用a Have a seat,please. Have a seat,please Please have a seat意思 Would you have a seat please? Please have a seat.还有同义句Please（ ） （ ） take a seat ,please. come in and have a seat中,为什么要用have a please have a seat（同义句转换） come in and have a seat ,please.） please have a seat的同意句有哪些? Please have a seat here.（改成否定句）