The ES of an activity that has only one predecessor is simply the EF of that predecessor.

ES (Earliest Start)

The earliest point in time at which a task, project, or activity can begin, taking into account the completion of preceding tasks and any necessary waiting periods.

EF (Earliest Finish)

In project management, it is the earliest possible point in time when a particular task or activity can be completed, considering the start time and the duration of the task.

Predecessor

A task or activity that must be completed before a subsequent task or activity can begin in project management and scheduling contexts.