Epics are significant initiatives that help guide the value stream towards the larger aim of the portfolio.
Primary artefacts used to define system behaviour in agile development. Stories are not requirements, they are short, simple descriptions of a small piece of functionality that delivers functionality to the customer/user. Estimation of work are captured using story points.
Task are not written in the same level of requirements as a user story. A task captures requirements in more of a technical nature.
The estimation of work is generally captured using time effort.
Sub tasks are used to break up an issue ( Story or Task) into separate pieces of work. Overall estimation using story points should be captured on the sub task.