He was.........when he thought better of it. Why "when"? How about "but"? When I was learning English words and idioms in a book, I found one incomprehensible sentence as follows: He was about to commit suicide by swallowing poison when he thought better of it. - Why use "when" instead of "but"? And I found another example in a dictionary like this - She was going to ask for more advice, but she thought better of it. - In here, being used "but". It is more understandable for me.