てから for talking about how long it’s been since a certain event “B, (ever) since A”
(てから for sequences, てから and たら comparison) )
--A (the thing that happened first chronologically) is something that happened in the past
--B (the thing that happened since the first action) is something that started when A happened and has continued since then OR a one-off action that happened sometime since A happened
For the grammar buffs out there, it works like the present perfect (“B has been X since A happened”) or present perfect progressive “B has been happening since A happened” tenses in English.
彼女はこの会社に入ってから２度昇進した。She has been promoted twice since she joined this company
東京へ来てからこんなに暑い日は初めてです。This is the hottest day I have had since I came up to Tokyo (this is the first time I’ve experienced a day this hot since I came to Tokyo)
父は手術をしてから体調が良い。My father has been in good shape since his operation.
--We can put in a duration: “It has been # (years). since A happened.” This is useful for saying the age of inanimate things, when you can’t use 歳/才（さい）(になる, が経っている is common here)
その図書館は建てられてから８０年になる。The library is eighty years old (lit. “it has been 80 years since the library was built
ここまでで幕が開いてから一時間経っている。About an hour has passed from curtain-up.
Notice that 経っている (x amount of time has passed) is the ている present progressive (present participle?), not the past (“経った”). The present-participle-or-past-tense game is an absolute nightmare for me but I’m noticing that the verbている works a lot like the present perfect tense in English (have seen, have gone, have eaten etc.)
--We can use it to ask a question
彼女と知り合ってからどれくらいになりますか。How long have you known her?
--Or we can put in ずっと (or というもの for extra emphasis) instead of a defined time to mean “B ever since A happened.”
彼は生まれてからずっとそこに住んでいる。He's lived there all his life.
身を立ててからというもの、あれは私にひどく寛容になって……」And ever since he made a success he was very generous with me.'
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