If I move a unit with a 'When I move' trigger to a battlefield and react with another unit using Ambush before the trigger resolves, will an enemy unit's 'when I defend, if an enemy unit is alone here' trigger still activate?

Ruling: No. The enemy unit's trigger will not activate because the 'alone' condition is checked at the moment of defender designation, and by that time there will be two enemy units present at the battlefield.

Sequence:

• You move your unit (e.g., Irresistible Faefolk) to the battlefield, triggering its 'When I move' ability
• While that trigger is on the chain, you play another unit (e.g., Rengar, Trophy Hunter) using Ambush as a reaction
• The chain resolves: your second unit enters first, then the move trigger resolves and moves the enemy unit to the battlefield
• Combat initiates and attacker/defender designations occur
• When the enemy unit becomes the defender, there are two enemy units present, so the 'alone' condition fails and the trigger does not activate

Nuances:

• Combat does not start until the chain is empty, so you can react to your own move trigger before attacker/defender designations occur
• This works whether the enemy unit was already at the battlefield or is being moved there by your trigger
• The 'alone' condition must be true at the exact moment of designation for the trigger to activate