Timing is everything

So, there are a number of questions that have to do with the "speed" of spells and abilities and I thought i'd take the opportunity to clear a few things up.  In MtG, the speed of something is actually used to tell, not how quickly something happened, but at what times you can do certain things.
When it comes down to it, there are really three "speeds" in the game (I'm not going to get into the nightmare that was Interrupts. Google them if you're curious.)
Sorcery speed, "fast effects", and mana sources.

Sorcery speed
When - During one of your Main Phases, when you have priority, and the Stack is empty
Things that operate at this speed - Playing a Land, Casting a creature spell, Rolling a Planechase die, Activating a Planeswalker's Ability, and interestingly enough, Casting a Sorcery.

Fast Effects/Instant Speed
When - when you have priority
Things that operate at Instant speed - Instants (duh), Activated Abilities, Casting a spell with Flash, Channel ability, Morphing, Cycling, and other similar abilities.

Mana Source
When - either when you have priority, when casting a spell or activating an ability that requires a mana payment, or whenever a rule or effect asks for a mana payment (even in the middle of casting or resolving a spell or activating or resolving an ability).
Things that operate as Mana Sources - anything that puts mana in a player's mana pool and doesn't have restrictions (See Lion's Eye Diamond).

Something that gets thrown in the mix to confuse things is that some things don't use the Stack. This means that they can't be responded to: Playing Land, cycling, morphing.
Example: Niel plays Shock, targetting Amy's morphed 2/2 creature. In response, Amy pays the mana and unmorphs it, revealing Akroma, Angel of Fury. Now, Niel has another Shock in hand, but when he gets priority, Akroma is already a 6/6.

Finally, there's paying costs of spells and abilities. This is done during right after you announce the spell or ability and put it on the stack. Fling is a good current example. If you chose to cast fling, you sac the creature as part of the cost to cast it. By the time its on the stack and priority is passed, the creature is already toast💀. They can't Doom Blade it in response or otherwise mess with the creature.

I hope this was enlightening. Let me know if you have any questions, or if I made a boo-boo.

So if I'm facing a 6/6 Grimgrin, the Corpseborn and I block with a 6/6, the opponent can sac a creature to get +1/+1 and it doesn't die?

So if I'm facing a 6/6 Grimgrin, the Corpseborn and I block with a 6/6, the opponent can sac a creature to get +1/+1 and it doesn't die?

I hate it when my gf remembers she can do that... 😭

Lol. Girlfriend duels are vicious. Lol

Your girlfriend should also remember when   Grimgrin, Corpse-Born attacks he can destroy a creature and get a +1,+1