Triggers
If something triggers when a player casts a spell, the trigger will resolve before the spell that caused the trigger.

* Managorger Hydra’s last ability will resolve before the spell that caused it to trigger.

* Once an ability triggers, it doesn’t matter what happens to the spell or permanent that caused it to trigger. The ability will remain on the stack even if the spell is countered, or the permanent that caused it to trigger is destroyed, before the ability resolves.

* If two or more abilities trigger for the same player at the same time, that player can put them on the stack in any order. The last ability that player puts on the stack will be the first one to resolve.

* If abilities trigger simultaneously for the active and non-active player, the abilities of the active player will be put onto the stack first, and the abilities of the non-active player on top of them. This means that the abilities of the non-active player will resolve first.

* Check to see what the triggering event is, to see how many times the ability will trigger:

* Kessig Forgemaster’s first ability triggers once for each creature blocking or blocked by it. The ability resolves and deals damage to that creature before combat damage is dealt. If that damage destroys a creature blocking Kessig Forgemaster, Kessig Forgemaster doesn’t become unblocked.

As opposed to

* Somberwald Alpha’s first ability will give each creature you control that becomes blocked +1/+1 until end of turn. It doesn’t matter how many of your opponent’s creatures are blocking each of your creatures.

In both examples the triggering event has something to do with becoming blocked.
For Kessig Forgemaster the event is triggered per “a creature”, so each blocking or blocked creature will cause the ability to trigger individually, in relation to that creature.
With Somberwald Alpha On the other hand, the event is “becomes blocked” as a whole. It will trigger as long as a creature becomes blocked, regardless of the actual number of blockers.
What’s also important to notice here is that while with the first example it’s just Kessig Forgemaster, for Somberwald Alpha the event is checked for each creature you control and not just Somberwald Alpha, so it will trigger each of your creatures individually.

Another example

* The ability triggers just once for each life-gaining event, no matter how much life is gained.

* A creature with lifelink dealing combat damage causes a single life-gaining event. For example, if a single creature with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), Cliffhaven Vampire’s triggered ability will trigger only once. However, if two creatures you control with lifelink deal combat damage at the same time, the ability will trigger twice.

* If an ability’s effect depends on the amount of something, check that number on resolution.

* Count the number of Undead Servants in your graveyard as the ability resolves to determine how many tokens to put onto the battlefield. If Undead Servant dies in response to its own triggered ability and is in your graveyard as that ability resolves, it will count toward the number of Zombies you get.

Enough with abilities for now – Let’s talk about casting spells.

Additional costs
* If a spell has an additional cost that requires you to do something such as to sacrifice a creature, discard cards, tap untapped creatures, etc’, players can respond to that spell only after it’s been cast and all its costs have been paid. Notably, no one can try to stop you from casting that spell by removing the creature you sacrificed, discarding cards from your hand, tapping your creatures, and so on.

Cost reductions
* If an effect reduces some portion of a spell’s mana cost, it only applies to what that reduction is – Something that reduces colorless mana can’t reduce the colored mana requirement of a spell, and vice versa.

* Cost reductions can’t make that portion of the mana cost to go below 0. In other words, you can’t pay minus mana, thus gaining mana to your mana pool.

* If there are additional costs to cast the spell, apply those before applying cost reductions.

* Cost reductions (as well as additional costs) also apply to alternative costs.

* Krallenhorde Howler’s first ability can only reduce the generic mana portion of a creature spell’s cost.

* Krallenhorde Howler’s first ability can reduce alternative costs such as madness costs.

Casting a card without paying its mana cost
* If you cast a card “without paying its mana cost,” you can’t pay any alternative costs. You can pay additional costs such as kicker costs. If the card has mandatory additional costs, you must pay those.

* If you’re casting a card that has {X} in its mana cost without paying its mana cost, you must choose 0 as the value of X.

Casting cards by other effects
* If an ability lets you play a card for a specific duration (such as “this turn” or “for as long as”), playing it follows the normal rules for playing that card. This means you must pay its costs, and you must follow all applicable timing rules.
* “Playing a card” means playing that card as a land, or casting that card as a spell, so in this case you could play a land card. You can’t play more than one land each turn unless some effect lets you, so if you’ve already played a land this turn you won’t be able to play another.

* You may play that card that turn even if Abbot of Keral Keep is no longer on the battlefield or under your control.

* Playing the card exiled with Abbot of Keral Keep’s ability follows the normal rules for playing that card. You must pay its costs, and you must follow all applicable timing rules. For example, if the card is a creature card, you can cast that card by paying its mana cost only during your main phase while the stack is empty.

* If you don’t play the card, it will remain exiled.

However, if an effect lets you cast a card as part of its resolution, ignore timing restrictions but not other restrictions.

* If the spell mastery ability applies, you’ll cast the two cards in order. The one you cast last will be the one that resolves first.

* You cast the instant and/or sorcery card(s) from your opponent’s library as Talent of the Telepath is resolving. Ignore timing restrictions based on the card’s type. Other timing restrictions, such as “Cast [this card] only during combat,” must be followed.

Mana with strings attached
Concluding with paying costs, some effects generate mana that you can only use for specific things. Needless to say, pay attention what you can spend the mana on.

* Mana produced by Ancient Ziggurat can be spent on any part of a creature spell’s total cost. This includes additional costs (such as kicker) and alternative costs (such as evoke or Worldheart Phoenix’s alternative cost).

* Mana produced by Ancient Ziggurat can’t be spent on activated abilities that put a creature card directly onto the battlefield, such as unearth or ninjutsu.