They should have gone earlier為虛擬語氣中的主句,表示與過去事實相反的情況,從句謂語動詞形式Had+過去分詞（過去完成時）,主句謂語動詞形式Should/would/could/might+have+過去分詞（現(xiàn)在完成將來時）
eg.If I had got there earlier,I should/could have met her.如果我早到那兒,我就會見到她.（事實：去晚了）
If he had taken my advice,he would not have made such a mistake.如果他聽我的勸告的話,就不會犯這樣的錯誤了.（事實：沒有聽我的話）
與過去相反,所以反問也用過去時.