Here are a bunch of assumptions and rulings and thoughts I have about D&D 5E, so that you won't be surprised or upset when something I say at the table contradicts the way you've read the rules and/or played in the past.
Dispel Magic
UNDER CONSTRUCTION
Assumption: This is a 3rd level spell, so should be powerful.
The general rule I use is that the Dispel Magic spell ends spells, not just effects.
Target
Casting dispel magic on either the caster of a spell or the target of that spell ends the spell.
Why? Because an ongoing spell (regardless of whether or not it requires concentration) requires a continuous connection from the caster to the target. Both are targets for dispelling. Additionally, both are targets for detect magic and identify.
TODO: I'm still thinking about that concentration bit. I might change my mind and decide that (some) nonconcentration spells have no further connection from caster to target, in which case the caster is not a valid target for dispel magic. On the other hand, spiritual weapon.
For example, a warlock has cast hex on an ogre. Casting dispel magic on the ogre ends the spell. The warlock does not get to retarget hex - the spell has ended.
Another example, a sorcerer has cast shadow blade. Casting dispel magic on the blade ends the spell. The sorcerer does not get to use a bonus action to reform the blade - the spell has ended.
Multiple targets
Casting a spell on multiple targets is still one casting of the spell. Dispel magic ends this one casting.
For example, a priest has cast bless on three targets. Casting dispel magic on any of the four creatures (priest, three targets of bless) ends the spell on all four creatures.
Another example, a druid summons four woodland creatures. Casting dispel magic on any of the creatures, or on the druid, ends the spell and unsummons all four beasts.
Yes, I know this is counter to Sage Advice. I think Sage Advice is wrong on this one.
Effects
TO DISCUSS:
So why would you ever target an effect? Why would you cast dispel magic on a shadow blade instead of the sorcerer?
Sometimes you only want to end one spell. If a sorcerer has shadow blade and mirror image active, casting dispel magic on the sorcerer ends both. Casting dispel magic on the blade only ends the shadow blade spell.
Sometimes you cant see, or don't know, who cast the spell. Perhaps the druid who summoned the beasts is hidden. Perhaps the wizard who cast heat metal on your armour ran away.
Illusions
Illusion magic is basically magical paint. Making an illusion of a door is no different, conceptually, to getting out some paint and brushes and making a picture of a door.
You could also think of it as magical CGI.
Subject matter knowledge
A character's ability to fool someone with paint (or a model, or a disguise, or camouflage, etc) is based on their painting skill but also on their experience with the subject matter. It doesn't matter how good an artist is, if they have never seen a cockatrice then they can't make a convincing picture or model of one.
The same goes for illusions.
If you don't know what uniform the duke's guards wear, you can't create an illusion of them (just like you can't disguise yourself as them).
Also, illusions do not have any sort of prescience or telepathy. If you don't know that the captain of the guard has a limp then your illusion of the captain will not have a limp.
As an aside, you can't create an illusion of a mirror and have it work like a mirror, any more than you can create a painting of a mirror and have it work like a mirror.
How do you deal with an illusion?
The easiest way is physical interaction. If it is a visible-only ilusion then touching it with something reveals its illusionary nature.
For example, a wizard has created an illusion of a wall and crouched down behind it. A PC fires an arrow at the wall and sees the arrow fly straight through the wall. They now know the wall is an illusion. It turns transparent, but only for them. They can yell to their allies "the wall is an illusion" but the illusion is not transparent to those allies until they themselves do something to physically interact with it.
You can also spend some time (an Investigation/Perception Action at least) studying it and looking for inaccuracies. How well has the artist crafted the illusion?
For example, a wizard has created an illusion of a rock. A PC is looking at it, trying to determine if its a real rock. The player rolls an INT check versus the caster's INT check. If the player wins then their character sees a transparent, badly drawn rock. If the player loses, their character sees a rock.
So how do I make my illusions effective?
The same way you make forgery or disguise or camouflage or deception more effective - do your research.
Phantasms
These are in the mind of the target so the rulings about illusions do not apply. The victim's mind fills in the details and makes it real.
In some cases the caster may not even know what the target is perceiving.
Invisibility
Seeing through invisibility cancels the invisible condition only for that observer. This is because some conditions are relative to an observer, not absolute.
If Wizardy McWizardface is invisible and Priesty McPriestface cas see invisible through some means, then Wizardy is not invisible to Priesty and the rules for unseen attackers and targets do not apply. Priesty is not at disadvantage to attack.
Wizardy is, however, still invisible to Barby McBarbarianface, and the unseen rules still apply. Barby attacks Wizardy with disadvantage.
Note that without some special ability, invisible creatures cannot see themselves. An invisible character can cast cure wounds on themself, but not healing word.
Hiding
Being hidden is more than being unseen and unheard - it means that observers do not know your exact location, or that observers don't even know if you are present or not.
An archer that everyone saw duck behind a tree cannot be hidden because observers know exactly where they are ("behind that tree!").
However if no-one saw the archer duck behind the tree (perhaps they concealed themself there before the observers arrived) then can attempt to hide.
An archer behind a long wall can attempt to hide, because observers don't know where along the wall the archer is going to pop up and shoot.
The Hidden condition is another relative condition. Barby can be hidden from Wizardy but at the same time not hidden from Priesty.
If Priesty can somehow communicate Barby's presence and location to Wizardy then Barby is no longer hidden from Wizardy.
Note that creatures and objects can be hidden in plain sight. We probably wouldn't use the word "hidden" though - we'd use "camouflaged" or "disguised".
Willing
Unconscious creatures cannot be willing. Being a "wiling target" requires active consent.
Metadata
I'm trusting you with the metadata.
That means I'm going to tell you what the AC of the monster is, and you tell me whether you hit or miss. I'm going to tell you (or at least give you clues) that the monster is resistant to fire or vulnerable to acid. I'm going to tell you how difficult the fight is going to be.
This gives you a lot of power when using metagame abilities like superiority dice and bardic inspiration dice. The rules say that you get to decide whether or not to use these items after rolling the die but before the DM tells you the result. Well, you're going to know the result as soon as you roll the die. More power to you.
Spellcasting
Casting a spell is obvious
Any intelligent observer can perceive the difference between someone using actual magic and someone saying nonsense words while waving their fingers.
Subtle casting is a very powerful ability.
Spellcasting Focuses
A focus is a crafted item, using special materials and techniques that focus the flow of magic. A wizard can't just grab any lump of crystal and use it as their arcane focus. A druid can't just rip a stick from the nearest tree and call it their druidic focus. A cleric can't just carve something into a piece of wood and call it a holy symbol.
They are, however, not actually magical. Any mundane crafter can create a focus provided they have the knowledge and skill (and materiels, of course).
Spell Books
Spell books are written in one of the languages the spellcaster knows. If you want to copy spells from another spellcaster's book then you have to understand the language they wrote it in.
You then have to spend time decipering the author's personal notation and converting it into your own notation. This is why copying spells has a time and gold cost (which is reduced when copying from one of your own books).
Spell Scrolls
Similarly, spell scrolls are written in one of the languages the crafter knows.
Note that copying a spell from a scroll into a spellbook has a chance of failing. This applies to all spellbooks (wizard, ritual caster, warlock, etc).
You must pay the time and gold cost on success or failure.
The scroll is consumed on success or failure.
I advise you to seek guidance.
Area-of-effect Spells with a Duration
In general, you can only be affected by such a spell once per turn.
That could be at the start of your turn or when you enter the area for the first time on a turn or when you finish your turn, depending on the individual spell. You can be affected multiple times in a round, however, with each affect happening on a different creature's turn.
For example: moonbeam, cloudkill, grease, cloud of daggers.
Also, these spells generally have no effect when cast. If a spell is an exception to this then it will say so in the spell description (for example grease).
Surprise
There is no such thing as a surprise round. Surprised is a condition that a creature gains if combat starts and the creature suspected no hostile moves from anyone.
You can surprise a creature by using stealth but also by using deception, camouflage, or other tactics.
Encumbrance
Is enforced.
I used to tell players that as long as they were being reasonable then I wasn't going to worry about it. Many players were not reasonable. In one case a single-digit-STR character was carrying five suits of heavy armour without the player taking any movement penalties.
This is why we can't have nice things.
Breaking up Actions
Generally not allowed.
One exception: You can move between multiple attacks in the Attack Action (Extra Attack or Multiattack).
Extra Attacks
When resolving multiple weapon attacks with the Extra Attack or Multiattack features, as well as spells that allow multiple beams (for example, eldritch blast, scorching ray), you must resolve the first attack before choosing a target for the next one.
For spells, you may not move between each ray or beam. Allowing movement leads to strange interactions with concentration and counterspell.
Magic missile is an exception. You must choose all targets before rolling any dice, then the missiles all hit simultaneously. Magic missile is not a "beam" spell - it is an AoE spell that can hit the same target multiple times. Multiple hits with magic missile trigger one concentration check and cause one failed death save.
Bonus Action Potion Use
You can drink a potion as an Action or as a Bonus Action (your choice).
Why? To encourage the use of potions without having to give up attacking (which is almost always the best use of action economy).
Feeding a potion to someone else is an Action.
The Rogue Fast Hands feature allows feeding a potion to someone else as an Action or as a Bonus Action (your choice).
Thrown Weapons and Extra Attack
If your weapons are somewhere easy to get to then you can draw and throw multiple items in one Action.
For example: daggers or darts in a bandolier across your chest, javelins which you have previously stuck in the ground in front of you.
Lucky (and similar) and Advantage/Disadvantage
Roll two d20s. Roll your Luck die. You may now choose to use your Luck die value in place of the value of one of the d20s. Now apply advantage/disadvantage.
Death Saves
If you want to add some tension to the table then roll them in secret. Don't tell the rest of the table (including the GM) what the result is until someone checks on your character's condition or attempts to heal them.
Feats
Timing of Shield Master and Polearm Master and similar
You choose the timing of the Bonus Action. If you Shove before attacking then you have locked in your intention to take the Attack Action.
Polearm Master
You can only take advantage of this if you use the weapon two-handed.
This feat also applies to tridents and pikes.
