Ask open questions when disagreements arise, rather than using statements or even accusations. Gentleness in asking those question is key: “How can I help?” is better than “Why are you banging on about that again?” 

Take some time alone to check out your own feelings and write them down. Are you feeling frustrated? Scared? Irritated? Jittery? Naming feelings is a powerful way to take away their sting and let them go. Feelings are transitory. They will pass and they don’t own you.

And try not to judge, this can be difficult when things are stressful, but it’s a wonderful skill. Communication problems are made worse if the other person jumps to conclusions, or butts in on what you’re saying, maybe with a hostile or quizzical expression. Be brave and bold. Go first. You can set an example.