Conventions of Composition Rule 204
Rule: Good writing shows (rather than tells), interests (rather than bores), and clarifies (rather that obfuscates). To-be verbs often work against all three of these goals. If you can avoid “is” in its many forms, do so. You have so many action verbs to choose from. Use them to keep your writing demonstrative, lively, and clear.
Note: Avoid "is because," "is when," and "is where." Not only are you likely using an adverbial clause as a predicate adjective (check out rule 151 for this), but you're also missing an opportunity to pack a punch with your verb choice. Avoid "there is" and "there are," as these constructions usually demonstrate sloppy thinking.
Examples:
Weak: I am happy today.
Better: I find myself singing a little ditty as I shower.
Weak: There are three examples of nice adjectives of this in the paragraph.
Better: Austen's use of "handsome," "clever," and "rich" demonstrates her assessment of the title character.
Weak: The reason Homer leaves the orphanage is because he is tired of his routine.
Better: Homer leaves the orphanage when he tires of his routine.
Weak: Creating imagery is when the author paints a picture with words.
Better: Imagery paints pictures with words. OR Authors use imagery to paint pictures with words.
Practice strengthening these sentences by eliminating to-be verbs:
Eden is clever and kind.
In Ebba's room, there is no room for more sports equipment.
The reason Sora didn't show up is because she had a big test the next day.
Imani knows that she is the smartest student in the class.
Resources for further explanation of weak to-be verbs:
Writing Commons' Eliminate "to be" Verbs
St. Louis Community College's Replacing To Be Verbs