Mouth breathing drops your tongue from the roof of your mouth, forcing your head forward and tilting it back to breathe. Posture exercises won't stick if you don't fix the breathing underneath, because your body prioritizes air over alignment.
I imagine if you've ever had a cold or something and your nose is all plugged up, what do you do? You breathe out of your mouth, right? You have to get air in. And so our body's going to prioritize breathing over posture, right? So you're going to start noticing that they're going to open their mouth to breathe. And eventually, long-term, that could turn into habitual mouth breathing. That's a problem. One reason it's a problem is because their tongue posture is going to change. This is kind of important because you're typically, your tongue is supposed to rest at the top of your palate when you breathe. And you imagine if you're mouth breathing, you can't breathe while your tongue is up in the top of your mouth, right? So long-term, you're going to adapt and start continuously breathing through your mouth, and then you adapt your posture. Likely it's going to be forward head posture and cervical extension. To get more air in, you're going to lean forward, tilt your head back to make that easier. We can treat that all day, every day, but if we never address the root cause of the problem, likely these, these adaptations are going to keep coming back because the body prioritizes survival over posture every day of the week.
