Reckless Imp — ruling 1

If you choose to pay the dash cost rather than the mana cost, you're still casting the spell. It goes on the stack and can be responded to and countered. You can cast a creature spell for its dash cost only when you otherwise could cast that creature spell. Most of the time, this means during your main phase when the stack is empty.

Reckless Imp — ruling 2

If you pay the dash cost to cast a creature spell, that card will be returned to its owner's hand only if it's still on the battlefield when its triggered ability resolves. If it dies or goes to another zone before then, it will stay where it is.

Reckless Imp — ruling 3

You don't have to attack with the creature with dash unless another ability says you do.

Reckless Imp — ruling 4

If a creature enters the battlefield as a copy of or becomes a copy of a creature whose dash cost was paid, the copy won't have haste and won't be returned to its owner's hand.

Reckless Imp : règles, fonctionnement et interactions (MTG)

Reckless Imp is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying This creature can't block. Dash (You may cast this spell for its dash cost. If you do, it gains haste, and it's returned from the battlefield to its owner's hand at the beginning of the next end step.)

How it works : Reckless Imp

Original English text — Wizards of the Coast / Scryfall

#1February 25, 2015· Wizards of the Coast
If you choose to pay the dash cost rather than the mana cost, you're still casting the spell. It goes on the stack and can be responded to and countered. You can cast a creature spell for its dash cost only when you otherwise could cast that creature spell. Most of the time, this means during your main phase when the stack is empty.
No other card is named in this ruling.
#2February 25, 2015· Wizards of the Coast
If you pay the dash cost to cast a creature spell, that card will be returned to its owner's hand only if it's still on the battlefield when its triggered ability resolves. If it dies or goes to another zone before then, it will stay where it is.
No other card is named in this ruling.
#3February 25, 2015· Wizards of the Coast
You don't have to attack with the creature with dash unless another ability says you do.
No other card is named in this ruling.
#4February 25, 2015· Wizards of the Coast
If a creature enters the battlefield as a copy of or becomes a copy of a creature whose dash cost was paid, the copy won't have haste and won't be returned to its owner's hand.
No other card is named in this ruling.