Some of the more complicated triggered abilities are those that include what is known as the ‘intervening if’ clause. These triggered abilities are generally written in the format “When/Whenever/At [trigger event], if [condition], [effect].” These condition followed by the ‘if’ functions as an addition trigger condition. If it is not true when this ability would trigger, then nothing happens, the ability does not trigger and nothing goes on the stack.
If this additional condition is also true, then the ability will trigger and go on the stack. However this condition is checked again when the ability attempts to resolve. If this condition is not true at that point, then the ability will do nothing. In other words these triggers have two different times that they can ‘fail’ to do anything.
Here is an example; the metalcraft triggered ability of Blade-Tribe Berserkers is a triggered ability that has an intervening if clause. When the Blade-Tribe Berserkers enters play this ability will trigger if you control three or more artifacts. If you do not, nothing will happen. If you do they this ability will trigger. This ability will then go on the stack and can be responded to. Then when this ability resolves, the game will check once again if you control three or more artifacts. If you do not, then the ability leaves the stack and nothing will happen. However if you do control three artifacts at this point the Blade-Tribe Berserkers will get +3/+3 and haste until the end of the current turn.