Modal+Verb. I can swim across the river.
Modal+Perfect.情态动词+完成时.You should have seen him off yesterday.你昨天本来应该去送他的.（对过去的虚拟）
Modal+Perfect+Progressive+Verb.情态动词+完成进行时He must have been listening to music when you knocked at the door.Or he would open the door for you.你敲门的时候他一定正在听音乐,不然他会给你开门的.（对过去的推测）