What it means
てからというもの means “ever since” or “ever since doing something.” It marks a starting point after which a situation, habit, or tendency has continued, often with an implication that the change has been lasting or especially noticeable. It usually follows a verb in the て-form and is commonly used in written or relatively formal narration.
Usage note
てからというもの is stronger than a neutral expression meaning simply “after.” The following situation is normally presented as continuing from that point onward, so it is not suitable for a one-time event that happened immediately afterward.
How to use it
日本に来てからというもの、毎朝五時に起きています。
Ever since coming to Japan, I have been getting up at five every morning.
日本に来てからというもの、毎朝五時に起きている。
Ever since coming to Japan, I have been getting up at five every morning.