The hens ______(lay)20 eggs by last Friday

had laid

laid 这边表示过去 强调过去 用过去时 上个星期五

The hens had laid 20 eggs by last Friday
句意：到上周五止，母鸡已经下了20个蛋了。
由于时间状语是by last Friday，所以应该用过去完成时。

The hens ______(lay)20 eggs by last Friday.
线上应填 had laid
解析：因为 last Friday 是过去时间，所以 lay 的过去式和过去分词都是 laid 。又因为是时间状语by last Friday ,所以应该是过去完成时 had done 形式，所以填 had laid .
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had laid 是过去完成时 （过去的过去)

填 had laid。句意：到上周五为止，母鸡已经下了20个蛋了。
（时间状语是by last Friday，所以应该用过去完成时）
希望对你有帮助，谢谢。