My father often tells me the things and persons whom he met

楼上的答案错了,正确的是whom 改为 that
因为先行词 是the things and persons （ 这些事和人）
即
My father often tells me the things and persons that he met in the past.
定语从句中只能用that 的情况：
1 先行词有序数词,形容词最高级时或本身是序数词,基数词,形容词最高级时只能用that
2当被修饰的先行词为 all ,any
.much ,many ,everything ,anything ,none ,the one 等不定代词时.
3 当先行词被the ony ,the
very ,the same ,the last ,little ,few 等修饰时
4 当先行词里同时有人和物时.
5 当主句是以who / which 引导的特殊疑问句时.

即有人又有物的时候，从句引导词用which

My father often tells me the things and persons he had met in the past.
要加had，whom去掉也可以，不影响。

把whom改成that就够了，因为先行词
the things and persons
在从句里做的是主语。而whom修饰的是宾语