Zone of Control rule of thumb: You can always move into the ZoC of your own forts. You can't move into enemy ZoC on one side and out the other without sieging down a fort, unless the first rule applies.
ZoC movement rules, the parts in italics are just comments to hopefully make it easier to remember:
1. Armies can always move from a non-fort province to an adjacent fort.
This is necessary, otherwise it would usually not be possible to reach most forts.
2. Armies can move to any province (whether in ZoC or not) that's on a distance of at most 2 from the return province going through only accessible, non-ZoC provinces.
This is what I think of as the "Zone of maneuvrability".
3. Armies can always move to and from a province within Friendly ZoC, which is the ZoC projected by forts that they both own and control. This is shown by green stripes on the ZoC mapmode - note that provinces in both Friendly and hostile ZoC show up as red even though they function as both.
Think of it as friendly forts in your land supporting your army movements.
4. Armies can always move to and from ships.
Think of it as "Dunkirking".
You can see from the fort mapmode where the ZoC (friendly and hostile, for any country) is, the only thing I'll point out here is the following:
1. ZoC never extends into provinces hostile to the country in which the fort is located, no matter who occupies it.
2. If you occupy a fort, your armies move as if it didn't exist. It still projects hostile ZoC against your enemies though, but no Friendly ZoC for you so you may not necessarily be able to move to that province even if you previously could due to rule #1.
Debunking a couple of widespread myths:
It is false that you can, as I said above, only move onto a fort or back to the province you came from.
It is false that you can always move to a non-ZoC province. It has to be within a distance of 2 from the return province.
It is false that you can always move into your own territory.
It is false that the rules prevents walking through a fort province. What matters is the ZoC, not where exactly the forts themselves are.
Here are two pictures from an England vs Burgundy war (return province Coast of Holland, where the 34 ships are) that you should be able to understand from the above. See it as an exercise:
And with the fort in Caen active:
I'm aware that there are weird situations that can appear when the ZoC changes, but that's a topic for another post.
