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Two

Say the short version.

Most of the difficulty is not in choosing words. It is in letting the words be brief enough that they stand on their own.

Five sentences that do the job

  • "I can't do that."

    Complete on its own. No reason required, and no reason means nothing to argue with.

  • "That doesn't work for me."

    Useful when there is nothing wrong with the request — it just is not something you are going to do.

  • "I need some time before I answer."

    Buys back the gap between the ask and the yes. Most automatic yeses live in that gap.

  • "I've already given you my answer."

    For the second, third and fourth ask. Repeat it unchanged rather than finding a better explanation.

  • "If this continues, I'm going to leave the conversation."

    Says what you will do, not what they must stop doing. Only say it if you are willing to actually stand up.

Softening usually makes it harder.

Apologies, long reasons and hedges are meant to protect the other person from disappointment. In practice they hand over material: something to solve, something to disagree with, something to try again next week. A shorter sentence is kinder to both of you.

  • "I'm so sorry, I feel terrible, but I really can't."

    "I can't do that."

  • "Maybe? Let me see how the week goes."

    "No, not this time."

  • "I can't because of work, and the car, and my sister is visiting…"

    "That doesn't work for me."

  • "Would it be okay if I maybe didn't?"

    "I'm not going to be able to."

When to say it

Earlier is easier than later, and calm is easier than mid-argument. If something has already gone wrong, you can raise it afterwards: "About last night — I'm not going to talk about my money again. I'll change the subject if it comes up."

You do not need the perfect moment. A slightly awkward one is fine. Waiting for a good one is often how a year goes by.

Keep it about your side

"Stop calling me late" asks them to change. "I turn my phone off after ten, so I'll pick up in the morning" describes what you will do. The second one is yours to keep whatever they decide.

This is not a trick of phrasing. It is the whole difference between a boundary and a demand — and it is why a boundary still works with someone who has no intention of cooperating.