Along with our Milestone Dependencies, you're able to ensure tasks are completed in a specific order through Task Dependancies.
In order to not conflict with higher-order Milestone Dependencies, a task cannot be dependent on a task tied to a different milestone.
Setting a task dependency
Select the task that must be completed in order to begin the task you have open
You'll notice your task is then locked and cannot be moved until the dependency task is completed.
Once the dependency task is completed, you may now further the dependent task.