のこととなるとWhen it comes to a particular topic
のこととなると introduces a topic that brings out a particularly noticeable reaction, attitude, ability, or change in someone. It often suggests that the person behaves differently when that subject is involved.
What the pattern highlights
The noun names a special area of concern or interest. The main clause then shows what becomes distinctive when that topic arises: someone grows enthusiastic, loses confidence, becomes unusually protective, performs exceptionally well, or otherwise changes noticeably. The contrast with the person's usual behavior may be stated directly or be clear from context.
Characteristic reactions
These examples make the person's usual behavior and topic-specific reaction easy to compare.
田中さんは無口だが、鉄道のこととなると、何時間でも熱心に語る。
Mr. Tanaka is usually quiet, but when it comes to trains, he will talk enthusiastically for hours.
Analyze普段は冷静な姉も、子どものこととなると、つい心配しすぎる。
Even my normally calm sister tends to worry too much when it comes to her children.
Analyze彼は仕事では大胆なのに、お金のことになると急に慎重になる。
Although he is bold at work, he suddenly becomes cautious when it comes to money.
AnalyzeThis uses the common variant.
Two similar-looking frames
Without the noun-attached のこと, general となると can mean that if a situation comes about, a certain consequence follows. That is a hypothetical situation, not necessarily a person's characteristic reaction to a topic. Meanwhile, について simply means “about” or “regarding” and does not imply any change in behavior.
More examples
兄は数字に弱いが、野球の記録のこととなると、細かい数字まで覚えている。
My brother is bad with numbers, but when it comes to baseball records, he remembers even the smallest figures.
Analyze部長はたいてい決断が早いのに、人事のことになると、急に慎重になる。
The department head usually makes decisions quickly, but becomes suddenly cautious when it comes to personnel matters.
Analyze彼女は自分への批判には平気だが、家族のこととなると、黙ってはいられない。
She is unbothered by criticism of herself, but when it comes to her family, she cannot stay silent.
Analyze祖父は普段あまり話さないが、昔の映画のこととなると、目を輝かせて話し始める。
My grandfather does not usually talk much, but when it comes to old movies, his eyes light up and he starts talking.
Analyze佐藤さんは温厚なのに、締め切りのこととなると、周囲にも厳しくなる。
Ms. Sato is mild-mannered, but when deadlines are involved, she becomes strict with everyone around her.
Analyze父は自分の健康には無頓着だが、母のこととなると、少しのせきでも病院へ行けと言う。
My father is careless about his own health, but when it comes to my mother, he tells her to go to the hospital for even a slight cough.
Analyze