Llanowar — ruling 1

A plane card is treated as if its text box included "When you roll {PW}, put this card on the bottom of its owner's planar deck face down, then move the top card of your planar deck off that planar deck and turn it face up." This is called the "planeswalking ability."

Llanowar — ruling 2

A face-up plane card that's turned face down becomes a new object with no relation to its previous existence. In particular, it loses all counters it may have had.

Llanowar — ruling 3

The controller of a face-up plane card is the player designated as the "planar controller." Normally, the planar controller is whoever the active player is. However, if the current planar controller would leave the game, instead the next player in turn order that wouldn't leave the game becomes the planar controller, then the old planar controller leaves the game. The new planar controller retains that designation until they leave the game or a different player becomes the active player, whichever comes first.

Llanowar — ruling 4

If an ability of a plane refers to "you," it's referring to whoever the plane's controller is at the time, not to the player that started the game with that plane card in their deck. Many abilities of plane cards affect all players, while many others affect only the planar controller, so read each ability carefully.

Llanowar : règles, fonctionnement et interactions (MTG)

Llanowar 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

All creatures have ": Add ." Whenever chaos ensues, untap all creatures you control.

How it works : Llanowar

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
A plane card is treated as if its text box included "When you roll , put this card on the bottom of its owner's planar deck face down, then move the top card of your planar deck off that planar deck and turn it face up." This is called the "planeswalking ability."
No other card is named in this ruling.
#2October 1, 2009· Wizards of the Coast
A face-up plane card that's turned face down becomes a new object with no relation to its previous existence. In particular, it loses all counters it may have had.
No other card is named in this ruling.
#3October 1, 2009· Wizards of the Coast
The controller of a face-up plane card is the player designated as the "planar controller." Normally, the planar controller is whoever the active player is. However, if the current planar controller would leave the game, instead the next player in turn order that wouldn't leave the game becomes the planar controller, then the old planar controller leaves the game. The new planar controller retains that designation until they leave the game or a different player becomes the active player, whichever comes first.
No other card is named in this ruling.
#4October 1, 2009· Wizards of the Coast
If an ability of a plane refers to "you," it's referring to whoever the plane's controller is at the time, not to the player that started the game with that plane card in their deck. Many abilities of plane cards affect all players, while many others affect only the planar controller, so read each ability carefully.
No other card is named in this ruling.

Interactions with other cards

These cards' text mentions Llanowar.

Freyalise, Llanowar's Fury

+2: Create a 1/1 green Elf Druid creature token with "{T}: Add {G}." −2: Destroy target artifact or enchantment. −6: Draw a card for each green creature you control. Freyalise, Llanowar's Fury can be your commander.

Abomination of Llanowar

Vigilance; menace (This creature can't be blocked except by two or more creatures.) Abomination of Llanowar's power and toughness are each equal to the number of Elves you control plus the number of Elf cards in your graveyard.

Llanowar Mentor

{G}, {T}, Discard a card: Create a 1/1 green Elf Druid creature token named Llanowar Elves. It has "{T}: Add {G}."

Llanowar Sentinel

When this creature enters, you may pay {1}{G}. If you do, search your library for a card named Llanowar Sentinel, put that card onto the battlefield, then shuffle.