ERDs are a fantastic tool and I'm sure once you get your head around them you will agree.
The relationships are always important to enforce. In your database the relationship between client and case is a 1 to many. This means that each case must have one and only one client but each client must have at least one case but they can have many. In this case each client should have a client_id which is the primary key and must be unique etc. this will be referenced in the case table as a foreign key so that the case table will have a client_id for each case. This will enforce the 1 to many relationship between the two tables.
As you can see most of the relationships in this diagram are 1 to many which is how a well designed database should be to enforce referential integrity. The only inconsistency with this is the relation ship between case and corporate case where the relationship is 1 to zero or 1. This means that a case may not have a corporate assigned to it and that if it does it must only have 1. In this case I would recommend using a PK in corporate case and link that to a FK in case.
If you need more information on how these relationships translate to English this page may help http://www.informit.com/articles/article.aspx?p=27281&seqNum=3
Good luck.