Generally speaking, a placement or diagnostic test is designed to be a measurement of your academic skills, so it is not something that you would "study" for in the traditional sense. Cramming or studying before taking an assessment test will yield inaccurate results, and could lead to you placing into class for which you might not have the skills to succeed.
However, taking time to refresh yourself in the general concepts can be helpful - especially if it has been a year or more since your last English or Math class. Furthermore, it's also wise to gain a familiarity with the format of a particular test when available. This helps avoid the feeling you've just jumped into a completely unknown situation.
The sample tests are designed to give you a familiarity with the tests. They do not necessarily reflect the difficulty level(s) of any test.
We have sample tests in the following subjects: