it wouldn't be difficult for a more careful person to find s

for a more careful person=if you(he 、she or anybody else) were a more careful person.
译文：要是你是一个细心的人,就不难发现这样的错误了.
言外之意,事实上：你不是一个细心之人.
所以用虚拟.
楼主注意一下,英文虚拟中经常出现介词词组代替If 从句的情况.
如：如果没有你的帮助,我就完不成这项工作了: without your help, i wouldn't have finished it.
without your help=if you hadn't helped me.