have的過去式和過去分詞分別是什么

　　There have been many changes in recent years.

　　近幾年發生了許多變化。

　　Call me if you have any problems regarding your work.

　　你如果還有什么工作方面的問題就給我打電話。

　　You seem to have set a new trend.

　　看來你們是開了一個新風氣。

　　No new SARS cases have been reported in the region.

　　該區沒有新增非典型肺炎病例。

　　These changes have occurred over a long time span.

　　這些變化經過了很長一段時間才形成。