The crowded population and busy traffic make people concern about the city.
I can't help but think of the days that I had in school.When I see this photo,I can't help but remember the days which I was in school.可以吗？When I see this photo,I can't help but remember the days which I was in school.这个把was改成had或者spent就可以了。因为days是名词，I was days，不符合逻辑。第一个的话，你尝试了主语从句，但是并不正确。因为你用主语从句的同时，有用到了which引导定语从句，结果就是主语从句缺少谓宾成分，建议改成：That the traffic is over crowded with people and is so busymakes people have more concerns on this city.