Define in your own words: impermanence, not-self, emptiness, suchness. Why does the Buddha stress awareness of impermanence, and how does it relate to other concepts?

Impermanence is the knowledge that everything around is is constantly changing. Nothing ever is static, or remains the same.

Not-self is how we as humans, should strive to exist - not limiting our identity by the use of labels, but rather recognizing that we are subject to change, evolve, and devolve. We aren't really anything. We just are, and we just exist. Anything else is a label that we or society gives ourselves.

Emptiness is going to be a bit harder for me to describe. It's not the feeling of being alone or socially rejected or isolated as we know this word to be. I think really it's recognizing that we are free of thoughts, free of desires and wants and anything else that can bring about dukha in our lives.

Suchness is being able to recognize what comes and goes, and noticing the little things and the big things all around us. To me, it's the essence of mindfulness.

I think impermanence is stressed so much because it really is the basis of everything. While things are here, they might not be here tomorrow, whether that is material possessions, loved ones, enemies, jobs, beliefs, even countries and cities. Because we as humans have a natural tendency to cling to all of these abstract and physical objects, labeling them as "mine" or "ours", we create dukha for ourselves, as if these objects go away, we are left with a sense of longing or bereavement. If we can recognize that while we have these things in the present moment, and we might not have them tomorrow, we can begin to free ourselves from dukha as we've recognized that things can change.