Reasoning for Soul Warden and Ajani's Pridemate interaction

Question

I play modern Soul Sisters and I understand that when I put an Ajani's Pridemate (or Archangel of Thune) down with a Soul Warden out, Ajani's Pridemate immediately gains a counter (and Archangel's trigger goes off as well).

I've had a few people at tournaments question this interaction and my understanding of how it works is a bit fuzzy, besides something regarding ordering the stack.

Why do the cards interact like this? It is more helpful if you can give me a response that I could explain to the person I'm playing against, so they also understand it.

Answer 1

It's pretty simple. Angel/Pridemate has to enter the battlefield to trigger Soul Warden. This means when the life-gain trigger resolves, the Angel/Pridemate will be able to see this and trigger themselves. So it's not actually a matter of specifically ordering the stack at all. It's actually just a chain of triggers.

As a list of steps (assumes no responses from anyone):

1. You have Soul Sister on the field
2. You cast Angel/Pridemate
3. All players pass priority
4. Angel/Pridemate resolves and enters the field
5. Sister's life-gain triggers because a creature entered your field
6. All players pass priority
7. Sister's trigger resolves and you gain 1 life
8. Angel/Pridemate's ability triggers because you gained life
9. All players pass priority
10. Angel/Pridemate gets a counter

Answer 2

To add to the existing answer and to avoid confusion: Ajani's Pridemate is not getting the counter immediately, the trigger goes to the stack first and your opponent still has a chance to react to it, for example by destroying it before it can get the counter.

I tried to understand why people at tournaments (who should have a bit more experience already) would question this and I assume you might have insisted on Ajani's Pridemate getting the counter the moment it reaches the battlefield.

Stack ordering only comes into play when you have both Ajani's Pridemate and Archangel of Thune on the battlefield at the same time, then you are allowed to say in what order their triggers are put on the stack (and then your opponent can react to them).

Answer 3

The warden has a triggered ability that lets you gain life when a creature enters the battlefield. So when the pridemate enters, the ability goes on the stack, eventually, it resolves. You gain a point of life which then in turns triggers the pridemate's counters ability.