Combat is a hugely important aspect of the game. It has a few simple rules and a few complex ones.
Each turn, you can attack your opponent with some or all of your creatures (not including those cast that turn). Once you decide which are attacking, the opponent gets to decide whether their creatures will block.
Multiple blockers can team up against one attacker, but a single blocker can’t block more than one attacker. A good visualization for this is to imagine the attackers running across a field that is very wide. The attackers are spread out from side to side, so each blocker has to choose which one they will engage and move to that spot. I use this to remember four things:
Any attackers that are not blocked deal damage to the defending player (or one of their planeswalkers), reducing their life total (or the planeswalker’s loyalty).
If an attacker is blocked, the attacking creature and the blocking creature deal damage to each other. If one or both of them take enough damage to equal or exceed their toughness, they go to the graveyard (a face-up pile next to the library).
The amount of damage that a creature deals is equal to its power, which is the number on the left at the bottom corner of the card. Often, creatures’ power and toughness will change based on what other cards are in play, so you have to keep track.
You and your opponent will have to do a fair amount of simple math during the game, so don’t hesitate to ask what a creature’s current power and toughness are. Certain modifications (+1/+1 counters) are traditionally represented with physical objects such as dice placed on the card, while others are usually just mentally added up after reviewing cards on the table.
Many creatures have abilities that change the way combat damage works - I’ll cover the common ones in a section on creatures. For now let’s review this sequence. First, one player chooses which creatures are attacking and taps them. Then, the defending player chooses blockers and usually moves them to touch the creatures they will block. Both players can cast spells after each step to try to impact the outcome - and then damage is dealt by all attackers and blockers.
For any creatures that don’t die right then, this damage is healed at the end of the turn; this is one way in which Magic is conceptually different from most other games. It is one of the ways in which Magic is an off-by-one game where a difference of one can be pivotal. For instance, a 4/4 doesn’t just stop one 3/3. A 4/4 could kill a 3/3 every single turn, and after the end of the turn it is as good as new (a 3/4 could kill multiple 3/3s, and a 4/4 could kill multiple 3/4s - so find ways to be off by one!).