Question

have had 一问

Answer 1

更新1:

先谢各位的解答，每位都解答得很详细啊

在你句子中的have had 无可争议是个现在完成式的虚拟词气(present perfect tense in subjunctive mood)
具' 曾经发生过' 的意思。所以整句可以改写为 What impact irrigation may have occurred
happened or taken place on the land-use pattern ? 问题就只是关乎到个 may 字而已。 may 是个动词
你也一定知是解作' 可以 '
' 可能 ' 之意
但它可作主动词(principal verb or main verb)
亦可作助动词(auxillary verb)。 作主动词(may--- used as a principal verb)： 1) It may rain.=It is possible that it will rain. 2) It may be true.=It is possibly or probably true. 3) You may go = You are permitted to go. 4) She may be right.等句子中的may字有本身的意义
所以是主要动词。 作助动词(may---used as an auxillary verb)： 1)He flatters that he may get promoted. 2)May you live happily together. 3)The man arrested may have had a history of child abuse. ' may '多在表达一个具目的
愿望等句子中作助动词。 就上述的结论
may 在你原句中没有had字就可以
虽然句子意义不大相同
但若连have也省去了就没有主动词
句子亦因而解不通了。
参考: nil
WorldDupFinal﹕ I will have been in USA for 5 years next month.不是题主想要的have had 连在一起的例子，可否另再举例？