〜てからというもの
ever since (and everything changed)
Verb て-form + からというもの
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てからというもの means "ever since", with strong emphasis on how much has changed: 子どもが生まれてからというもの、生活が一変した, "ever since our child was born, our lives have completely changed".
It's like て以来 (N3), but more emotional and literary. The first half is a turning point, and the second describes a new state that has continued ever since, often a dramatic difference from before.
The second half can't be a single event or a future plan; it's a situation or habit that has lasted: 元気がない, 毎朝散歩するようになった, 毎日が新しい発見だ.
It's common in essays, memoirs and storytelling.
Examples
子どもが生まれてからというもの、生活が一変した。
Ever since our child was born, our lives have completely changed.
彼女と別れてからというもの、彼は元気がない。
Ever since he broke up with her, he's been down.
犬を飼い始めてからというもの、毎朝散歩するようになった。
Ever since I got a dog, I've gone for a walk every morning.
日本に来てからというもの、毎日が新しい発見だ。
Ever since I came to Japan, every day has brought something new.
禁煙してからというもの、体の調子がいい。
Ever since I gave up smoking, I've felt great.