Infer Mallam is told to "shut up" whenever he tries to speak, so that eventually
"the tea seller learned to swallow his words." Why is Mallam treated in this way?
What does this behavior towards Mallam add to readers' understanding of both his
character and Ghanaian society?

Mallam was treated that way due to the fact he belongs to the low-class society. In Ghana, there are different social classes: The upper class where the rulers, nobles and similar people are part of it. The other is the middle class and the last one is the low and working class where Mallam is part. They have different rights, that's why Mallam does not have the right to speak before the higher class.