- Teachers, who give homework, are unpopular.
How is this sentence a fragment? And how to correct it?
This is not a fragment: it is a complete sentence with a subject and finite verb. It is also grammatically correct. However, you will probably want to remove those commas. With commas, it is a descriptive relative clause, which means “all teachers (who, by the way, give homework) are unpopular”. That is probably not what you mean. Without the commas, it would be a defining relative clause, which means “only those teachers who give homework are unpopular; other teachers may very well be popular (emphasis on “who give homework”)”.