Creatures are not brought into play tapped, but they have summoning sickness preventing them from tapping to attack or to tap to activate any abilities unless they have the keyword "haste".
Yes, you can regenerate a tapped creature in Magic: The Gathering by paying the regeneration cost, even if the creature is tapped. Regeneration prevents the creature from being destroyed the next time it would be destroyed that turn.
When a creature in Magic: The Gathering can't be regenerated, it is destroyed and cannot be saved or brought back to the battlefield.
No, unless they have haste.
When a creature in Magic: The Gathering dies, it goes to the graveyard and can potentially be brought back to the battlefield. When a creature is exiled, it is removed from the game entirely and cannot be easily returned.
In Magic: The Gathering, when a creature is regenerated, it means that if it would be destroyed, it instead becomes tapped and removed from combat. This allows the creature to survive the destruction effect. Regeneration typically requires the payment of a specific cost, such as mana or tapping the creature itself.
No, a creature is not considered a spell in Magic: The Gathering.
No, a planeswalker is not considered a creature in Magic: The Gathering.
In Magic: The Gathering, the mechanic "regenerate target creature" allows you to prevent a creature from being destroyed by paying a cost. When a creature is targeted for regeneration, if it would be destroyed that turn, instead it is tapped, removed from combat, and all damage is removed from it. This allows the creature to survive and stay on the battlefield.
No, note it says 'Untap target nonlegendary creature", not "Untap target tapped nonlegendary creature." Most of the spells that steal control like this, will allow you to untap the creature if it's tapped. But they're all still fine to be used against creatures that are originally untapped too.
No, in Magic: The Gathering, you can only block one creature with one creature.
No, creatures with summoning sickness cannot be tapped in Magic: The Gathering.
Yes, in a game of Magic: The Gathering, you have to tap a creature to use it to block an attacking creature.