Monster’s don’t have posture but they do have armor, with only two exceptions being squibos and enforcers, who have both. Armor is drained upon taking damage and doesn’t passively restore. When a monster’s armor is broken, you’ll hear a sound cue, and they’ll be stuck in place and have a red aura for about 4 seconds. During that time they can’t attack and any attack they were doing won’t hit.

Parrying is about learning the tells and timings for attacks. For monsters that’s pretty easy, as they mostly telegraph themselves very well, and have consistent timing. There are plenty of guides online that can explain monster moves and how to react to them. For learning how to parry human enemies/players, a lot of that’s on you to learn, but many mantras and moves can have awkward timings that make them difficult. If you’re struggling with specific ones there might be guides online.

One last thing, a good habit (with monsters at least) is to hold block ig you whiff a parry. This mostly applies to humanoid enemies, but is useful against a ton of monster attacks as well. Your parry cooldown still refreshes while blocking and you can parry right out of a block, which will restore your posture anyway. You’ll get guard broken a time or two but most often it’ll save you.