it's our custom to greet people by giving them flowers请分析相关语法

it's our custom to greet people by giving them flowers请分析相关语法
it's our custom to greet people by giving them flowers请分析相关语法

it's our custom to greet people by giving them flowers请分析相关语法
it形式主语
our custom表语
to greet people by giving them flowers真正的主语