[QUOTE=Unregistered;539902]Which is correct?
1a. There are different kinds of rules.
1a is correct...not much confusion here. Good job! ;)
2b. The students' books
2b is correct here if you are referring to the books of the students.
When do we have to use 's' plural possessive in this context?
Basically...you use the plural form to indicate that more than one person or thing is being referred to. But there are variations of how the possessive is used.
Instead of listing them all here...I will point to you to a few good resources that can explain it in more detail to help you really nail this down.
There are some exercises for you to practice with at the end of the third link there.