Jack said it was just what was worrying him most.
“杰克说这就是最让他担心的”.
主语Jack
谓语said
宾语从句it was just what was worrying him most.这又是一个强调句it was ……that/what,what was worrying him most是“最让他担心的”之意.what作主句it was just what was worrying him most的表语,同时作后面从句的主语.