It is impolite to start eating first if there are older people at the table.
if状语从句修饰哪部分，it是指to 后面全部嘛？it是指to start eating first if there are older people at the table嘛？
【分析】（1）if状语从句修饰 to start eating first （2）it 指to start eating first if there are older people at the table
