Translation:It rained last week.
I wrote the same thing and then I got the answer 'It rained last week' I wanted to throw my keyboard out of the window lol.
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.
了 is way overused on this course. It gives the impression that 了 is automatically used like V+ed in English. Overuse of 了 is a common problem for foreigners learning Chinese .... I know it was for me!
BTW Keith, thanks for helping out on these boards and posting so many useful comments. It's great to have input from a native speaker; believe me, it's greatly appreciated.
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
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.
Can the person who downvoted explain why it is not a good answer? It seems pretty reasonable to me. (disclaimer: I am not a native speaker, just learning)