The main aid in finding a relationship is to note that a relationship is usually a verb.
A relationship between two entities is either 1-1 (one to one), 1-n (one to many) or m-n (many to many), depending on its maximum cardinality. An example of a 1-1 relationship might be between Student and Locker, if each student can have only one locker and students do not share lockers. If one of these conditions is broken, the relationship becomes 1-n; and, if both are broken, the relationship becomes m-n.
The membership of an entity class in a relationship is either mandatory (obligatory), in which case each entity instance in the class must be in the relationship, or optional (non-obligatory).
A relationship may be unary or recursive; this means that both the entities belong to the same entity class. For example, one author cites another; authors can even cite themselves.
Participation of a weak entity in at least one relationship is mandatory.