I grabbed this sentence off NHK news easy, https://www3.nhk.or.jp/news/easy/k10012139481000/k10012139481000.html .
The sentence is, 国立成育医療研究センターと国立がん研究センターは、２０１６年と２０１７年,約８００の病院などでがんだとわかった若い世代について調べました。
The following sentence uses がんだとわかった as well. I believe this to be がん as in cancer, followed by the copula then the quotation particle and 分かる. I'm having some confusion as to what the subject is though. First off, I think there are two subjects. The first being the two research centers marked by は. They are doing the investigating (調べました).
２０１６年と２０１７年,約８００の病院などで is marking the time and place. 2016/2017, at approximately 800 hospitals.
Then, for the modifier of 世代, "がんだとわかった若い世代, I'd get something like "It is understood to be cancer young generation" or more fitted for English 'the young generation who are known to have cancer." The だ is for cancer as a state of being so there should be something like Zeroががんだとわかった. What's the zero or unsaid subject here though? 人たち? It?
I'm mainly confused at why the だ is used with an unsaid subject. I know it happens all the time but especially since this clause is modifiying 若い世代, it seems a bit redunant to me. If you take the だ off, then there is no state of being inferred for the young generation with cancer. It happens in the following sentence too and is modifying 人. がんだとわかった５万７７８８人.
Sorry for the rambling but if anyone can help me understand this it would be much appreciated. Thanks