There is one argument every relationship has. It is not big, it is not dramatic and it never gets resolved. The dishes. The flat nobody cleaned. The "you never do anything". You row, you make up, you breathe out in relief - and three days later you are standing in the same spot saying the same words. Sonia Díaz Rois, a coach and anger management specialist, offers a line sharper than any accusation about a partner's character: "What wears a relationship down most is not that you argue, but that you always argue about the same thing."

First, the timing. According to her, we almost always argue at the worst possible hour. Late at night, tired, after a hard day, while doing three things at once. "With the battery below the minimum and without really being present, our most rehearsed version comes out more easily" - which is a nice way of saying we react automatically, exactly as we reacted the previous hundred times.

Second, and this is where it gets interesting: the thing you are arguing about is usually not the problem. "Sometimes the argument starts with the dishes, with the mess, or because somebody did not finish something, but the real problem may not be that obvious." Behind it, she says, sits "take me into account", "value what I do", "hear me", "help me" - or the exact opposite, "right now I need some space and quiet".

The problem is we do not say it like that. Instead of "I cannot manage today, could you", we say "you never do anything". That is the difference between a request and a reproach, and a reproach does one single thing: it pushes the other person into defence. Díaz Rois compares it to learning a foreign language - you know what you want to say, you cannot find the words, so you start breaking the language of anger: louder, more sweeping, with reproach. And the message comes out completely distorted.

From there comes the pattern that hurts most: I chase, you run. One asks, insists, repeats, because they want to be heard. The other feels that as pressure and withdraws. And the more one withdraws, the more the other insists. Both think they are simply reacting to the other - when in fact they are dancing the same dance. "And if we want to change that dance, somebody will have to start practising a different step."

How do you tell a healthy argument from one that is eating the relationship? With one question, she says: what are we having this conversation for. Not why it started - that is usually clear - but what we are continuing it for. If the only goal is to prove you are right and get the other to admit it, the odds of ending up in the same place or worse are high. A healthy argument does not mean agreeing on everything; it means you can explain what you think while being ready to hear what they think.

And here comes the part that convicts most of us: making up is not a solution. "We reach such a level of tension that afterwards the only thing we want is to stop feeling bad. We apologise, we make up and we breathe out. But whatever took us into the argument is still unresolved." Peace is not the same as agreement - something anyone who has been in a long relationship knows and rarely admits.

Her practical advice is unexpectedly simple: agree on a keyword. Hers is "tortilla". When you see the conversation is not going to produce anything good, you say the word and you stop. The condition is that the word is agreed in advance, in a calm and ideally funny moment - and, more importantly, that you agree when the conversation comes back. Stopping must not mean sweeping it under the carpet. "You do not learn to swim while someone is drowning," she says. In other words: if you learn to talk about what bothers you while you are calm, you will manage it when you are angry too. Anger does not want to shout - it shouts only when nobody listened earlier.