You are looking at the wrong part. It isn't the "if" phrase that defines it. It is the verb that follows it. Easy examples first:
- "If you win a challenge in which Daenerys Targaryen attacked alone, move all gold token from the losing opponent's gold pool to your gold pool." (CS-Dany)
Obviously passive, right? Why? Not because of the "if you win" part, but because the verb telling you what to do ("move") is an active, one-time verb. You win the challenge and "move" happens. If someone cancels the "move" part (and there have been generic, non-triggered ability cancels before), it doesn't reassert itself, even though you still "won the challenge" and other "if you win a challenge..." effects might still be happening. Or how about:
- "If you do not win dominance, pay 1 gold or discard (Whatever) Refugee from play." (all 7 Refugees)
Again, obviously a passive, right? And again, because of the verb telling you what to do ("pay" or "discard") when the "if" condition is met, not because of the "if" itself. On the other side of the coin:
- "If you are not running an Agenda, The Red Viper gains: (text)" (KL-Viper)
Obviously constant, right? But this time, because the verb telling you what to do ("gains") is itself a constant, ongoing action or state (in this game's context). Similarly:
- "If you have no power on your House card, Beric Dondarrion cannot be killed or discarded from play." (Berric Dondarrion)
Again, obviously constant because the verb is one that describes an ongoing action or state.
So you see, you identify whether something is a passive or a constant effect based on the active vs. ongoing nature of the verb after the "if" statement. That, after all, is what the effect is actually doing.
Now, I can already hear you saying that there is a distinct difference between the above examples and the "if you control X" restriction/condition that we have on the Hatchlings. And you are right. There is. The difference is that the type of verb in the "if" statement matches the type of verb in the effect part. "If you win X" matches the active, one-time idea of verbs like "move," "pay," and "discard." On the other hand, "If you are not running" and "If you have X" match the constant, ongoing action or state of the verbs they are paired with ("gains" and "be"). However, the "if you control" condition is paired with both active, one-time verbs and constant, ongoing action or state verbs. For example:
- "If you control at least 1 Wildling character, treat attached location as if its printed text box were blank." (Climbing Spikes)
That verb there ("treat") is one which describes an ongoing state, so this would seem to be constant. The example here is pretty much the same as the KL-Viper or Berric. But what about:
- "If you control Ser Barristan Selmy, discard Arstan Whitebeard from play (cannot be saved)." (Arstan Whitebeard)
Here, the verb, "discard," is one of those active, one-time verbs. So do we interpret this as a passive effect, the way we did when the verb "discard" showed up on the Refugees? The fact that it is specified to be "cannot be saved" pretty much tells us that we should interpret this as a passive. If this was truly constant, you wouldn't need to say "cannot be saved" because the timing would prevent it, right? (That's the argument on the Hatchlings through this thread, isn't it?) Or, if you don't like the "save" example because the rules don't say anything about not being able to save vs. constant effects, only that you cannot cancel, then how about:
- If you are the only player to control The Iron Throne, you automatically win dominance (cannot be canceled). (DotN Iron Throne)
Aren't we looking at the same thing here? The verb "win" is one of those active, one-time verbs, implying that this should be interpreted as a passive effect that resolves and is over with, not a constant effect that just keeps on going. If this were a constant effect, and constant effects cannot be canceled, why put "cannot be canceled" in there?
The confusing part here is that "if you control..." is that type of "constant play restriction" that is usually, but not always, paired with constant effect verbs. It does end up having an impact on how the effect is resolved. Earlier, I likened it to a terminal effect. For illustration, the thing to do would be look at the full resolution of the general flowchart on p. 18 of the FAQ. To whit:
When a passive activated (Step 4.I), you have the opportunity to save/cancel (Step 4.II), which would then impact the resolution of the passive (Step 4.III). Most of the time, you are done with this. The one-time, active "if X" play restriction has already been factored into once. However, when the "if" phrase is using a constant, ongoing state verb like "control," the play restriction reasserts itself in Step 4.IV, effectively triggering the passive again. And again. And again until the one-time, active verb of the card ability resolves and there is no way to resolve it again successfully. You can only win dominance once. You can only discard Arstan once. You can only attach the Hatchlings as duplicates once. (And only 1 of those 3 examples even remotely involves the moribund rules exception.)
Note that all of that is exactly what happens in a "kill at 0" terminal effect situation. The "if the character's STR is 0" is a constant play restriction that would normally be paired with a constant, ongoing verb for a constant effect. But here, paired with an effect verb of "kill," it reasserts itself, even after a save/cancel (as the definition of a terminal effect tells us it does). So the situation with something like Arstan, the Iron Throne, and the Hatchlings is exactly parallel: an "if" phrase with an ongoing verb and an effect verb that resolves just once when that "if" condition is met.
So that's how I tell. I look at the verb of the effect part to see whether it is an active, one-time verb that can be resolved only once. If so, that's a passive effect. On the other hand, if it is an ongoing action or state (like "gains" or "cannot be,"), it is a constant effect. Then I look at the "if" phrase that places the condition/play restriction on the activation of that effect verb. When the "if" condition verb matches the effect verb, no problem. When they don't match, it is usually an ongoing condition verb matched with a one-time effect verb. Those are passives, but terminal in that they reassert themselves in Step 4.IV of the resolution of that passive. Looking at the Hatchlings with their "if you control" ongoing condition verb and "attach" one-time effect verb, they are the latter type of effect; passive, but terminal in that it reasserts itself.
And no, I don't expect people to get all of that quickly and easily from a single reading of the rules. That's where the experience comes in. And quite honestly, in the end it doesn't matter much whether it is a passive that reasserts itself or a straight constant effect because the final result is the same - there is nothing in the game that can stop it from happening.
Actually many things in this game is already confusing, that's why I want to improve rules document.