it left my mother very sad and resentful

LEFT BEHIND

After Gates died, he left his son his fortune.

it left my mother very sad and resentful.
It - 代表這語句之前發生了的事件. 假如是:
Our dog was killed by a traffic accident last night, it left my mother very sad & resenful.
left (p.t of leave) = 帶給, 留給, 留下, 有時可解做 導至
left (n) = 左, 左方

其實這句之前，一定發生了一些事情；而這些事情遺留下來的後果，就是使我母親非常悲傷與憤怒（怨恨）。所以，在這裏，left是留下的意思，但所留下的並不是實物，而是事情的後果。

希望你明白！

Cambridge dictionary :
If something leaves something else, a part or effect of it stays after it has gone or been used.
EXAMPLE: Far from improving things the new law has left many people worse off

Left 在這裡解作 令(某人)只剩下一種(的感覺)
Left 是 Leave 的過去式

example:
The death of my father left my mother very sad and resentful.
因為爸爸的死訊, 剩下了媽媽一人感到非常悲哀和忿恨.