Tutorial Step 9: Connect Tasks with Dependency Lines
Of course, most tasks require some other task to be finished before they can begin. You can represent this by connecting tasks with dependency lines. There are several types of dependency, but the most common is a Finish→Start dependency.
Select two tasks which need to be connected, and click the Connection button 
in the toolbar. A Finish→Start dependency line appears between the tasks, starting from the task which comes first in the outline.
in the toolbar. A Finish→Start dependency line appears between the tasks, starting from the task which comes first in the outline. 
As you connect tasks, they automatically reschedule to respect the dependencies.