In Magic the Gathering, I cast Song of the Dryads on an opponent's commander. It is now a colorless forest land. My opponent has an Angelic Field Marshal on the field which gains power if you control your commander. Is the colorless forest land still my opponent's commander? Does my opponent get the Lieutenant bonus?
It is still your opponents commander and they will get the Lieutenant bonus from the Marshal. This is because being a commander is a property of a card, it doesn't matter if the characteristics of the card have changed at all it will still be their commander.
903.3. Each deck has a legendary creature card designated as its commander. This designation is not a characteristic of the object represented by the card; rather, it is an attribute of the card itself. The card retains this designation even when it changes zones.
Example: A commander that’s been turned face down (due to Ixidron’s effect, for example) is still a commander. A commander that’s copying another card (due to Cytoshape’s effect, for example) is still a commander. A permanent that’s copying a commander (such as a Body Double, for example, copying a commander in a player’s graveyard) is not a commander.