Most people think that 939 is a bad class just because they can't tank/rush, but that's the point. They are a ambush type, and not a rusher, which is why there are other classes like 096 and 173, who do those things. So the first thing you wanna know is to not rush. Yes, it's fun to rush. Yes, you kill people while rushing. And Yes, you do have a lot of hp. But that doesn't matter. 939 is easy to kill, even with the hp and ahp, which is why you have to learn to not rush. If they are last, then yeah, you can most likely go for it, but if they aren't, then you are gonna waste a lot of hp.
Second, if camping checkpoint or a hallway is against server rules, then 939 is gonna be a lot crappier. 939's main thing is camping and ambushing, and yeah, you can ambush someone while running around, but it's a lot easier to do it when sitting at a good hallway to ambush.

Abilities:
As seen in the pictures above, 939 has a unique vision. They can see players from pretty far away through the walls (around 15.5 meters). The only way to combat 939 vision is slow walking (or crouching) which can be used by holding C or whatever button you keybinded to it. 939 also can't see you if you are not moving. However, 939 can see the other scps wherever. Another thing is that 939 can see everybody on surface, no matter if you are slow walking. There is also no way to turn off this vision.
This also means that scp 939 can see people wearing the hat (as long as they are moving).
Scp 939s footsteps are also silent as long as you don't sprint.
They can also talk to humans by pressing V (don't worry, you can still communicate with the fellow scps with Q).
Be aware, though, that even though they are blind, they are still affected by flash bangs.

DAMAGE:
Scp 939s do 50 damage per bite and also give you amnesia for 3 seconds. Amnesia reduces your vision quite a bit.

HEALTH + HUME:
Both 939s have 1800 health and 600 hume/ahp.

MOVEMENT SPEED:
Walk: 3.4 m/s
Sprint: 5.3 m/s

SPAWN LOCATION:
They spawn in heavy containment in their chambers. To get up, they need to open one of the two doors, climb the stairs and then open another door, then viola, you are out.

OTHER INFO:
Paired together, the 939s are stronger, as if they both land a hit on a human, they will usually kill it.
Since scp 939-89 is bigger than the other, it will be harder to try and hide, as you might have something sticking out (ex: face, leg, etc).
Meanwhile, scp 939-53 does not have this problem, as they are smaller and can hide more easily unless they press themselves completely against the wall/door they are behind.

NOTE:
Not all servers have scp swap, but I do recommend one that does so you aren't stuck with a class you hate or are bad at.

And now you know the basics of playing scp 939! Now get out there and enjoy the murderous life of a doggo.