There are plenty of situations where we can say something that is grammatically correct, but, socially, is not quite the right thing! For example, if we need to stop talking to a person who is talking a lot, because we have something else to do, it is much better to say, "I mustn't keep you" than to say, "You are talking too much! Let me go!"
Both expressions are correct English, but only one is polite.
"I mustn't keep you" means, you are busy and I must allow you to go; it is often used with this meaning. But it is also used when somebody means, I must go!
Other general situations which happen again and again in life are thanking people, complaining, saying "no" to something, talking about the weather, and interrupting somebody politely.
The things to say can change too, depending on how well we know the other person, or how formal or informal the situation is.
In this lesson, we learn and discuss suitable phrases, and if there is time, we can role play them too.