The thesis analyses relevant literature to define the terms of epistemic and deontic modality. Specific attention is paid to the semantics of MUST and SHOULD with its syntactic patterns. The practical part of the thesis compares and contrasts epistemic and deontic occurrences of MUST and SHOULD on diachronic samples of British fiction that stylistically represent literature written for children. The results show particularly at a shift from deontic to epistemic usage of MUST that might be rooted in the dynamics of the socio-cultural paradigms.