Speaking to your partner with respect and appreciation is important, especially in times of conflict.

The most successful couples, according to psychologists, regularly express devotion, understanding, and contrition.

Here are the five phrases that couples who have a strong connection use most, according to experts.

1. "Thank you."

John and Julie Gottman are psychologists who have studied more than 40,000 couples in search of answering the question: What makes love last?

The one phrase they say all successful couples use often is "thank you."

"A thriving relationship requires an enthusiastic culture of appreciation, where we're as good at noticing the things our partners are doing right as we are at noticing what they're doing wrong," they wrote for CNBC Make It.

This is especially true for small, everyday acts, they say.

"Tell them why that small thing is a big deal to you: 'Thank you for making the coffee every morning. I love waking up to the smell of it and the sounds of you in the kitchen. It just makes me start the day off right,'" they say.

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2. "Help me understand this."

Harvard psychologist Cortney Warren says successful couples don't avoid conflict, they just better navigate it.

We often assume we know what our partner is saying when in reality they might be expressing something more nuanced or totally different.

"If your partner reacts to a situation in a way that you don't understand, telling them that you want to know them better is key to resolving conflict and bonding at a deeper level," Warren wrote for CNBC Make It.

3. "I can forgive you. Can you forgive me?"

When you're in the heat of an argument or at the end of one it can be hard to express forgiveness. Do it anyways, Warren says.

"Studies have shown that couples who practice forgiveness are more likely to enjoy longer, more satisfying relationships," she says.

4. "I am committed to you."

"Being in a relationship is a choice," Warren says. "Reassuring your partner that you're still choosing to be with them and to work through challenges will help create a sense of safety and stability."

This might seem obvious, but communicating your promise to one another can help you and your partner feel validated.

5. "I like you."

"The healthiest couples don't just love each other, they like each other, too," Warren says. "Loving someone is an intense feeling of affection; liking is about seeing them for who they are and acknowledging the attributes you enjoy about them."

Even if you and your partner aren't arguing, remind them that you like them.

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