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xia4 yu3 le 下雨了: xia4 下 drop, yu3 雨 rain, le 了 a particle to indicate the action has occurred.下雨了it rained.
In Chinese, there is no change of verb tenses. So, if you want to say something that has happened, it is a good way to use the particle le 了 at the end of the sentence. For example: Wo3 kan4 dian4 ying3 le 我看电影了 I have watched movie, wo3 chi1 le 我吃了 I have eaten or ta1 lai2 le 他来了 He has come.