If an opponent's Merchant in base has Cleave played on it, then moves into a battlefield I control, can I play Gust on the Merchant during the 'When I move' trigger to return it to hand before it gets +3 attack?

Ruling: Yes, you can play Gust on the Merchant in response to its 'When I move' trigger before it gains +3 attack from Cleave. Combat cannot begin (and roles cannot be assigned) while items are on the chain.

Sequence:

• Opponent plays Cleave on Merchant (Merchant gains +3 when it attacks)
• Opponent moves Merchant into your battlefield
• Merchant's 'When I move' trigger goes on chain
• You can respond with Gust to return Merchant to hand
• Chain resolves before combat begins

Nuances:

• 'When I attack' triggers work differently - they only trigger when a unit becomes an attacker (after combat begins and roles are assigned)
• The standard move itself is not denied; combat simply cannot start until the chain is fully resolved