Learn more about Grammar:
Adjectives: An Easy Guide with ExamplesAdverbial Clauses: What Are They?AdverbsAnalogy: Definition & Meaning (with Examples)ArticlesBad AdverbsClauses: Definition, Meaning, and How to Use ThemConjunctionsCoordinating Conjunctions: What Are They and When Should You Use Them?Dangling ModifiersDeclarative Sentence: Examples + MeaningExaggerationHomophones: Definition and ExamplesHow to Use Subordinating ConjunctionsInfinitivesInterjections: Definition, Meaning, and ExamplesIntransitive Verb: Definition, Meaning, and ExamplesNounsParticiples PluralsPrepositional Phrase: What Is It & How to UsePrepositionsSplit Infinitive: The Complete Guide (with Examples)Subordinate Clause: Definition, Types, and ExamplesThe Complete Guide to Transitive VerbsTransitions: A Complete Guide (with 100+ Examples)Types of VerbsVerbsWhat is a Pronoun? Rules and ExamplesWhat Is Symbolism in Writing?Word ClassesWhy should you not overuse words like "believe" and "think"?

When you're writing fiction, using believe or think means you're telling your reader something instead of showing. Consider the following examples:
She couldn't believe her eyes.
She stumbled to a stop, gaping at the sight.
In the first example, you're telling your reader what the character is feeling. In the second example, you're showing astonishment through the character's actions.
When you're writing non-fiction, believe or think turn the focus on the writer instead of the subject of the sentence. You don't want to draw attention to yourself as the writer, so drop the believe and think and make a bold statement.
I think the candidate has a pertinent point on environmental issues.
The candidate has a pertinent point on environmental issues.
The first sentence is discussing what you, as the writer, think. The second sentence makes a bold statement that focuses on the candidate.