Does Stifle stop a landfall trigger if cast after the land enters the battlefield?

When a land enters the battlefield, any landfall triggered abilities trigger and are placed on the stack. Critically, there is a window after the land has entered — and the trigger is on the stack — where a player may cast instants like Stifle. This is because triggered abilities use the stack just like spells do (CR 603.3).

Stifle counters a triggered or activated ability (CR 509.2 governs priority after a trigger is placed; Stifle's text targets a triggered or activated ability on the stack). Once Stifle resolves targeting the landfall trigger, that trigger is countered and has no effect (CR 700.5 — 'counter' removes something from the stack without it resolving).

The land has already entered the battlefield by this point, so Stifle does not undo the land entering — it only prevents the landfall effect from happening. For example, if you control a Lotus Cobra, the landfall trigger fires when a land enters, but the mana addition hasn't happened yet. You cast Stifle targeting the Cobra's trigger; the trigger is countered and you get no mana, even though the land is now on the battlefield.

Timing is important: once the landfall trigger has already resolved, Stifle cannot retroactively counter it. You must cast Stifle while the trigger is still on the stack and before it resolves.