The three main categories of the nursing theory are:

1. Grand nursing theories: These are broad in scope and provide a comprehensive framework for understanding nursing practice. They are often complex and abstract and seek to explain the nature of nursing and its role in healthcare. Examples of grand nursing theories include the theory of human caring by Jean Watson and the self-care deficit theory by Dorothea Orem.
2. Middle-range nursing theories: These are narrower in focus than grand nursing theories, and provide a more specific and practical framework for understanding nursing practice. They often deal with specific patient populations or health problems and can be tested and applied through research. Examples of middle-range nursing theories include the chronic sorrow theory by Georgene Gaskill Eakes, Mary Lermann Burke, and Margaret A. Hainsworth, and the theory of uncertainty in illness by Merle H. Mishel.
3. Practice nursing theories: These are focused on the practical aspects of nursing care, and provide guidance for the delivery of care in specific settings or situations. They are often developed by nurses who work in clinical practice and are based on their experiences and observations. Examples of practice nursing theories include the theory of holistic comfort by Katharine Kolcaba and the clinical decision-making model by Patricia Benner.