Some triggered abilities have an “intervening if” clause—that is, they are written in the form of “When/Whenever/At [trigger event], if [condition], [effect].” If you choose such an ability with Trigger Happy, you ignore the trigger event, and the ability will go on the stack even if the intervening if condition is not fulfilled, but the ability will still check the intervening if condition as the ability tries to resolve. If the condition is not fulfilled at that time, then the ability won’t resolve. For example, Tatyova, Steward of Tides has the ability “Whenever a land enters the battlefield under your control, if you control seven or more lands, [effect].” If you choose that ability, it will go on the stack regardless of how many lands you control. However, as the ability tries to resolve, if you don’t control seven or more lands, the ability fails to resolve and the effect doesn’t happen.