Study the phrases about relationships.
  
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To be in a relationship - to be romantically involved with someone;

To be just good friends - to not be romantically involved;

To keep in touch with - to keep in contact with;

To lose touch with - to not see or hear from someone any longer;
  
Love at first sight - to fall in love immediately you meet someone;

To pop the question - to ask someone to marry you;

To see eye to eye - to agree on a subject;

To settle down - to give up the single life and start a family;

To strike up a relationship - to begin a friendship;

To tie the knot - to get married;

To be well matched - to be similar to;

To work at a relationship - to try to maintain a positive relationship with someone.

Examples:

We are just good friends.

Do you believe in love at first sight?

Sally is in a relationship with Mike.

It is essential to work at a relationship.