If Cathy blames her history teacher for her poor performance on a history test she is most likely engaging in: Self serving bias
Self serving bias refers to a way of thinking that always perceive ourself in a positive manner. People with self-serving bias often willing to over look their own mistakes in order to avoid the feeling of failure, which cause them to constantly blame other people for their own mistake.
In short and in general, formal language is the language you would use when talking to your boss or employes and informal language is the one you use when talking to a friends
