If they left now,they would arrive in good time.
從條件從句看,“如果他們現(xiàn)在離開”,用一般過去時(shí),表示與現(xiàn)在事實(shí)相反,又因?yàn)榻Y(jié)果主句的主語是they,那么應(yīng)該用would+動(dòng)詞原形的形式,用虛擬語氣.
與現(xiàn)在事實(shí)相反的虛擬語氣的規(guī)則是：
條件從句形式為：If (we,you,she/he,they)+動(dòng)詞過去式（be用were）
結(jié)果主句形式為：I（we）should
You would
She/he would + 動(dòng)詞原形
They would