Sth about modal verb,

i saw a unit in advanced grammar in use which is about the usage of could and can,i don't understnad the part of theoretical possibility,here are the examples from the book,i can't really tell the differences between them:
1)It could be expensive to keep a cat.(=if we had one,it could or it may not be expensive)
2)It can be expensive to keep a cat(=it can be,and it somethimes is)

Also,i don't understand the book saying "we use can't,not couldn't,to say sth is theoreticallt or actually impossible".could anyone explain it to me?And,what are the differences betewwn "can't have+pp" and "couldn't have +pp"

THX!!!!!