Asgorath
Explorer
Right you can move before and after your action. Now please show me where the rules state when any action ends.
You see the point is that if you don't actually know when an action ends then you can't determine if you are still allowed to move on your turn.
I'm not claiming there isn't an end to an action. I'm claiming the rules leave that end undefined.
I think it's actually pretty simple, and most actions are simply instantaneous.
Attack - JEC says it ends when you've taken all your attacks.
Cast a Spell - Many/most of these are instantaneous. For example, Scorching Ray has a duration of "instantaneous" despite it having 3 or more rays that you roll to hit for individually.
Dash - Instantaneous. You take this action, and your speed doubles.
Disengage - Instantaneous. You take this action, and movement no longer provokes OAs.
Dodge - Instantaneous. You take this action, and all attacks against you have disadvantage.
Help - Open for debate.
Hide - Instantaneous.
Ready - Instantaneous.
Search - Open for debate, I'd argue not instantaneous.
Use an Object - Open for debate, probably not instantaneous.
There's no need to argue about breaking up a Dash, Disengage or Dodge action for example, because the action is instantaneous and the effects last for the duration. While those effects are in play, you can do other things you have access to, such as bonus actions or movement.