How to ensure one event does not come before another
Ensuring that one event cannot come before another in your timeline is a useful mechanism, especially when dealing with items that are dependant on one another.
The best way to ensure that one event does not come before another is to implement a dependency or constraint.
Creating a Dependency
Click on one of the events in the dependency you are creating. Then, click on the dependencies and constraints tab:
You will then see the option to make this event "Block" another event, or be "Blocked by" another event.
To make this event block another event, click the + icon next to 'Block', and choose the event in your timeline that you want to be blocked by the event you currently have open in the Inspector.
To create a situation where this event is blocked by another event, click the + icon next to to 'Blocked By' and choose the event you wish for this event to be 'Blocked By'.
Adding A Constraint
Another way to ensure that one event cannot come before another is to add a constraint. In the tab marked by the button you will see the 'Date Constraints' section.
If the item you currently have open is the event you wish to come after another item, you would choose the option that this item's 'Start' 'is after start of', then you would implement the item that it cannot come before in the box next to 'Item'.
Alternatively, if this is the event that you want to come first, then you can choose that the 'Start' and 'is before start of' options, and then choose the item that this date must come after.