Stunned 1 and Slowed 1

[Lanathar]

I am really confused about these conditions . They seem the same - reduce actions by 1

What is the difference?

If a monk inflicts Stunned 1 on their turn and then it becomes the monster turn is it just simply the monster can only take 2 actions instead of 3? Making it the same as Slowed 1?

Only difference I can spot is not reactions inbetween ?

[Blave]

Slowed 1 can last multiple rounds. And as you said, a stunned creature can't take reactions.

[shroudb]

plus, dont they stack? a slowed 1 stunned 1 creature should have 1 action only, no?

[Unicore]

Technically it is not just reactions, but also free actions. A stunned creature can't take any actions until their condition clears.

[Staffan Johansson]

Slowed usually has a duration, so you're not just "Slowed 1", you're "Slowed 1 for a round" or "Slowed 1 for a minute" or the like. Every turn you take within that duration, you lose one action.

Stunned X instead causes you to lose X actions. Technically, the condition means you can't take actions, and at the start of your turn each point of Stunned cancels one of your actions until you either don't have any more actions or any more Stunned.

Stunned can also have a duration, in which case you can't take any actions at all for that duration. That's less common. But you never mix a value and a duration on Stunned - you're either "Stunned 3" or "Stunned for 1 round", but never "Stunned 3 for 1 round". Slowed on the other hand always has both a value and a duration.

In the particular case of being "Slowed 1 for 1 round" and "Stunned 1", the main difference is that you can't take any reactions until it's your turn. There are also some things other people can do to interact with the condition – not very many, but e.g. the Medic archetype has a feat that lets you reduce someone's Stunned condition. Slowed, on the other hand, is usually applied by some outside effect that can be counteracted somehow (e.g. dispel magic vs a slow spell).

Edit: Since you mentioned monks, I wonder if you are looking at the 2nd level feats and trying to determine whether to take Brawling Focus (crit specialization with unarmed attacks, which lets you inflict Slowed 1 for a round on a crit) or Stunning Fist (which lets you inflict Stunned 1 if you attack the same target with both Flurry attacks and at least one hits). Stunning Fist is more powerful on the face of it, but you should note that the stunning effect carries the incapacitation tag, which means it's pretty much useless versus higher-level foes. Critical Specialization is a slightly weaker effect, and won't happen as often, but at least things will rarely be outright immune to it.

[Staffan Johansson]

shroudb wrote:

plus, dont they stack? a slowed 1 stunned 1 creature should have 1 action only, no?

They don't stack, as described in the Stunned condition. If you start your round with Stunned 1 and Slowed 2, the action you lose to Stunned still counts toward Slowed, leaving you with a total of 1 action.

[Abyssalwyrm]

Slowed condition usually have time, and if so - it applies only on start of your next turn. Stunned applies effect immediately, but subtracting condition severity with every subtracted action.
So if someone stunned you as a reaction during your turn - you potentially will lose all your remaining actions.