Spellcasting Rules

NB: I highly recommend that all spellcasters read the five pages concerning spellcasting in the Player's Handbook, p201-205. Below are some of the common points of confusion. 
Bonus Action Spells, PHB p.202:
A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven't already taken a bonus action this turn. You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action. 
NB: This is also true of a spell cast with a sorcerer's Quicken Spell ability. Such a spell counts as having a casting time of a bonus action, and so the only other spells that can be cast in the same turn are cantrips.
Concentration, PHB p.203:
Some spells require you to maintain concentration in order to keep their magic alive. You can end concentration at any time (no action required). The following factors can break concentration:
- Casting another concentration spell. You can only concentrate on one spell at a time.
-Taking damage. Whenever you take damage while concentrating on a spell, you must make a constitution saving throw to maintain your concentration. The DC equals 10 or half the damage you take, whichever number is higher. If you take damage from multiple sources, such as an arrow and a dragon's breath, you make a separate saving throw for each source of damage. 
- Being incapacitated or killed. You lose concentration on a spell if you are incapacitated or if you die.
Longer Casting Times, PHB p.202:
Certain spells (including spells cast as rituals) require more time to cast: minutes or even hours. When you cast a spell with a casting time longer than a single action or reaction, you must spend your action each turn casting the spell, and you must maintain your concentration while you do so. If your concentration is broken, the spell fails, but you don't expend a spell slot. If you want to try casting the spell again, you must start over.
Readying a Spell, PHB p.193:
When you use the 'ready' action to hold a spell for a certain trigger, you cast the spell as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration. If your concentration is broken, the spell dissipates without taking effect.
For example, if you are concentrating on the web spell and ready magic missile, your web spell ends, and if you take damage before you release magic missile with your reaction, your concentration might be broken.
Casting Spells with a Verbal Component while Underwater, from this Sage Advice tweet: 
"No rules prohibit using verbal components underwater. However, keep in mind that if you're talking, you are not holding your breath."
i.e. If you are holding your breath, you can cast 1 spell with verbal components while underwater. After this, you immediately begin drowning. Creatures that can breathe water (naturally or through magical means) are unaffected by this.
Casting a Spell with Material Components, PHB p.203:
A character can use a spellcasting focus (if there class is capable of doing so) or acomponent pouch in place of the components specified for a spell. 
If a cost is indicated for a component, a character must have that specific component before he or she can cast the spell.
If a spell states that a material component is consumed by the spell, the caster must provide this component for each casting of the spell.
Category:Mechanics and Rules