Old-Growth Troll — ruling 1

If you control but don’t own Old-Growth Troll, you (not its owner) will return it to the battlefield when it dies. You’ll choose which Forest you control it will enchant as it returns to the battlefield.

Old-Growth Troll — ruling 2

If there’s nothing for Old-Growth Troll to legally enchant, it remains in its owner’s graveyard.

Old-Growth Troll — ruling 3

If a token is a copy of Old-Growth Troll, it won’t return from its owner’s graveyard.

Old-Growth Troll — ruling 4

If a nontoken permanent that's a copy of Old-Growth Troll dies while it is a creature, it will return to the battlefield as an Aura with both its own normal abilities and the two new abilities granted by old-Growth Troll. The resulting permanent will also retain its name, colors, and supertypes, but it will be an Aura enchantment with no other types or subtypes.

Old-Growth Troll : règles, fonctionnement et interactions (MTG)

Old-Growth Troll 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

Trample When Old-Growth Troll dies, if it was a creature, return it to the battlefield. It's an Aura enchantment with enchant Forest you control and "Enchanted Forest has ': Add ' and ', , Sacrifice this land: Create a tapped 4/4 green Troll Warrior creature token with trample.'"

How it works : Old-Growth Troll

Original English text — Wizards of the Coast / Scryfall

#1February 5, 2021· Wizards of the Coast
If you control but don’t own Old-Growth Troll, you (not its owner) will return it to the battlefield when it dies. You’ll choose which Forest you control it will enchant as it returns to the battlefield.
No other card is named in this ruling.
#2February 5, 2021· Wizards of the Coast
If there’s nothing for Old-Growth Troll to legally enchant, it remains in its owner’s graveyard.
No other card is named in this ruling.
#3February 5, 2021· Wizards of the Coast
If a token is a copy of Old-Growth Troll, it won’t return from its owner’s graveyard.
No other card is named in this ruling.
#4February 5, 2021· Wizards of the Coast
If a nontoken permanent that's a copy of Old-Growth Troll dies while it is a creature, it will return to the battlefield as an Aura with both its own normal abilities and the two new abilities granted by old-Growth Troll. The resulting permanent will also retain its name, colors, and supertypes, but it will be an Aura enchantment with no other types or subtypes.
No other card is named in this ruling.