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Corwin, I'm tired of this. You keep stating that the DM can just ignore the stated action of the player, I think that's a very adversarial role.
As far as "the DM decides when", all I can think of is that you're referring to the ability checks section of the rules.
Page 58 basic rules:
Ability Checks
An ability check tests a character’s or monster’s innate
talent and training in an effort to overcome a challenge.
The DM calls for an ability check when a character or
monster attempts an action (other than an attack) that
has a chance of failure. When the outcome is uncertain,
the dice determine the results.