【表语完全倒装Seatedinthefrontaretheleadersoftheschool.此句中的seated是过去分词吗?系动词为何要用are?此句和Sittinginfrontaretheleadersoftheschool.这句的句子结构是?Fastenedtothepolewasthenationalf】

　　表语完全倒装

　　Seatedinthefrontaretheleadersoftheschool.此句中的seated是过去分词吗?系动词为何要用are?此句和Sittinginfrontaretheleadersoftheschool.这句的句子结构是?Fastenedtothepolewasthenationalflag.的系动词又为何是was?