Translation:It rained last week.
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I wanted to answer those questions about 了. Though I know how to use it, I still have difficulty to explain it. Yet I checked several grammar articles about it and all of them said this V 了 N pattern is no good unless it is followed by another clause. e.g. 上星期下了雨，感觉凉快多了。Thus Duo's sentence here is probably not a good reference.
It appears that "下了雨" is a "separable verb" or can at least behave like one (and unlike one) since "下" is also a standalone verb meaning "to go down" and "雨" can be the noun "rain". According to the grammar at "https://resources.allsetlearning.com/chinese/grammar/Separable_verb#Where_to_put_.E4.BA.86.2C_.E8.BF.87.2C_.E7.9D.80", "了" should be placed between the verb proper ("下") and the noun ("雨") in such cases.
This example might be thought of as "poured" ("下了") "rain" ("雨").
I don't see "下雨" on any list of separable verbs though; "下雪" (to snow) is shown as one at https://app.ninchanese.com/stage/grammar-lesson/103/97, however, so it seems reasonable to assume that "to rain" has the same property.
Another environment where V了N is typically used (always assuming a situation that the situation calls for the use of 了 at all of course) is when the N is something more complicated than a bare noun – for example a time frame or an object with attributes. For example I agree that *昨天下了雨 is weird at best, but 昨天下了一場