In the mid 14th century the linguistic division between Norman French speaking nobles and English speaking commoners started to break down.
In Roger de Mohante's time as Rector of St Nicholas, in 1362 the Statute of Pleading was passed and English became the language of the courts.
In civil law the problems were typically secular affairs such as boundary disputes, non-payment of debts, ownership of property, individual and community rights, and so on. It is easy to see why lawyers prospered. The way to riches and wealth lay in advocacy.
A clerk would have need to study not only the law but also theology, as demonstrating a sound knowledge and understanding of theology would open the way to preferment within the church.