have + 动词过去分词 = 现在完成时,表示一个动作发生在过去,到讲话时已经结束了.主语是第三人称单数的时候,have 自然要转变为 has .
I have had breakfast.have breakfast 是固定用法,即吃早饭,have作为动词,过去分词形式是 had ,套入公式即为 have had.
He has bought a car.bought 是 buy 的过去分词,has 是 have 的第三人称单数形式.
I have been to Switzerland.主语是 I ,所以要搭配 have .需要搭配 have 的,除了 I 外,还有 we 和 they ；需要搭配 has 的,有 he 、she、it.