What’s wrong with this sentence?
“We can choose either to believe his words or reject them.”
This sentence has faulty parallelism. The correlative conjunctions either … or join items that are not in the same grammatical form.
To see whether a sentence has faulty parallelism, look at the words immediately following either and or. In the quoted sentence, the first item is to believe,
an infinitive preceded by to; the second item is reject, an
infinitive not preceded by to. To correct the sentence, add to before reject.
We can choose either to believe his words or to reject
them.
If you need help with parallel constructions, Focus on Grammar: Parallel Constructions is the book to read. When you finish the book, spotting and correcting faulty parallelism will be second nature to you.
