First, congratulations on going through the available training materials!
The ability to make some edits to a sketch when not directly editing the sketch allows experienced users to make changes to a model much faster than otherwise possible. I use this ability in almost every model!
However, this is also why we almost always recommend fully constraining and dimensioning sketches.
If you don't need to edit a sketch, hide it from view 😉
When accidentally deleting anything from a sketch, often a subsequent feature in the timeline turns yellow and creates a warning message. Undo the operation if the deletion was not intended.
If it was intended, fix the root cause and the right away. Don't ignore Fusion's warnings 😉