"it showed me that" is a valid and usable part of a sentence in written English. You can use it to show that something has served as an example or demonstration of a certain fact or truth. For example: "The incident at the store it showed me that people can be incredibly selfish when it comes to getting what they want.".
It showed me that really dark, tough subjects could be handled in a children's book, and that a book like this could pack a huge emotional punch.
"It showed me that it's actually over".
"It showed me that people want to talk.
"It showed me that things can be changed," Steven said.
It showed me that a writer could be kind and still be a master.
It showed me that the world was a big, fascinating, complex place filled with possibility.
It showed me that, one, I could get there, and two, where I needed to improve".
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Ha Thuy Vy
MA of Applied Linguistic, Maquarie University, Australia