Patience, understanding and not take things to heart.
I found it so hard when my wife had post natal depression. I couldn’t understand what was going on. Why she was being so unreasonable. It hasn’t gone away and my eldest is 7 now.
I still don’t fully understand it, and don’t think I ever will.
But I can read it better. Know when to comment and when to stay quiet.
Support wise. Just need to be there. Try and pick up the slack when she is low. If wants to sleep I take the girls out.
Frustrating and horrible thing to live with.