@GunnerMcGrathDash can or cannot be taken twice in a turn, once as an action and once as a bonus action (for rogues and monks)? A rogue using Cunning Action and a monk using Step of the Wind can use Dash twice on a turn (action + bonus action).
— Jeremy Crawford (@JeremyECrawford) February 10, 2015
The rules say: “When you take the Dash action, you gain extra movement for the current turn. The increase equals your speed, after applying any modifiers. With a speed of 30 feet, for example, you can move up to 60 feet on your turn if you dash.”
So for a Monk with 40ft move due to unarmored movement, you can:
Move 40ft
Dash 40ft
Ki point to dash 40ft
For a total of 120 movement speed.
Same goes for a rogue who runs 90 feet due to dashing twice.
What if the rogue is also hasted? would it be possible to dash 3 times?
Yes, with haste you can dash three times, and at double speed. So rogue would get 60 (standard movement) + 60 (action dash) + 60 (haste action dash) + 60 (cunning action dash) for 240ft of movement in one turn.
It’s considerably more effective on a monk though, even changing the base move speed to 40 changes the final speed to 320ft. At level 6, your max is 360ft. Level 10; 400ft. Level 14; 440ft. Level 80; 480ft. If you’re a Tabaxi, that can be doubled for one turn to 960ft.
It can go much higher with magic items, but this is already really fast.
Are you sure added movement speed is not a modifier?
So, if my total speed is 30ft and I move 20ft. Does my dash give me and additional 30′ or an extra 10?
It would give you an additional 30ft (whatever your base speed is) as additional movement. (How much/little of your Movement you use has no bearing on the movement provided by Dash, Step of the Wind, or similar, unless a specific feature explicitly says that it does.)