てからというもの means "ever since ~". It goes after the て-form of a verb, and the second half describes a change that set in at that point and has lasted to now.
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Ever since I came to Japan, every day has felt fresh.
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Ever since my child was born, my life has completely changed.
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Ever since I saw that movie, I've been thinking about its world.
What comes after it
The second half is a state or a habit that has carried on since the turning point. A single event that happened afterwards takes plain .
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Ever since we broke up, I haven't been able to concentrate on work.
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After I came to Japan, I climbed Mt. Fuji.
- × 日本に来てからというもの、富士山に登った → 日本に来てから、富士山に登った
てからというもの or て以来
also means "ever since" and attaches the same way. It states the fact; てからというもの makes the change sound like something the speaker felt deeply.
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I haven't seen him since we graduated.
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Ever since my grandmother passed away, my grandfather has lost his energy.
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It belongs to writing and to speech that looks back – an essay, a speech, a story about your life. In conversation, てから with says the same thing more plainly.
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I've felt well ever since I moved.
Lingaku