For what it's worth -- "Be" verbs are mainly linking verbs; they take predicate nominatives and predicate adjectives, and are used to form the passive voice.
He is a general.
He is happy.
The policeman arrested the thief.
The thief was arrested by the policeman. (Used to form passive voice)
To improve writing, try to avoid using too many "Be" verbs. For they have a tendency to weaken your writing. Instead choose stronger verbs that add flavor to your writing. An excellent exercise -- construct paragraphs without using one "Be" verb. After you make a habit of this, you will notice a vast improvement in what you write.
Of course, I'm no expert in this field. But, I have stolen this advice from experts.