Does returning a creature from the graveyard count as casting?

Yes. A creature spell is considered a creature spell no matter where it was cast, including from the graveyard (according to C.R. 601.2a, a player moves a card “from where it is to the stack” as the first step of casting it).

Is entering the battlefield a triggered ability?

The most common zone-change triggers are enters-the-battlefield triggers and leaves-the-battlefield triggers. 603.6a Enters-the-battlefield abilities trigger when a permanent enters the battlefield.

Do undying creatures come back tapped?

Similarly, when Geralf’s Messenger dies (with no counters on it), undying will return it to play (tapped, because the card says so), with a +1/+1 counter. Because it entered the battlefield again, its enters-the-battlefield ability will trigger, and the opponent will lose 2 life.

When you return a creature from the graveyard does it have summoning sickness?

Defy death returns creature from graveyard to battlefield, does the creature have summoning sickness? Yes. It won’t be anything unless it or the card has/gave Haste.

Can you respond to something entering the battlefield?

1. Correct. You can respond to the creature spell while it’s on the stack, but once it resolves, it enters the battlefield immediately.

Do tokens with undying come back?

Tokens can gain undying, and the ability will trigger if a token is put into a graveyard. However, like all tokens, it can’t return to the battlefield and will cease to exist as a state-based action before the ability would go on the stack.

Can a creature with summoning sickness crew a vehicle?

Summoning-sick creatures can crew vehicles, just as they can tap to activate other abilities like convoke. You’ll need to crew a vehicle every turn you wish to use it, so make sure you have some untapped creatures around.