Whatever he says, I will not __________ him.

The correct answer is believe because modal auxiliary verbs like 'will' must always be followed by a bare infinitive verb form.

• Step 1 (Grammatical Rule): The phrase 'will not' utilizes the modal auxiliary 'will' to establish the future tense or intent. Modal verbs are strictly paired with the base form of a verb without any endings or conjugations.


• Step 2 (Application): The bare infinitive form of the verb is 'believe', which completes the future negative statement properly.


• Incorrect Options:
  
  
  
  • believed is incorrect because it is the past tense or past participle form, which cannot follow a modal verb.
  
  
  • believing is incorrect because the present participle continuous form requires a form of 'to be' (e.g., 'will not be believing').
  
  
  • believes is incorrect because it is conjugated for a third-person singular subject, whereas the subject here is the first-person pronoun 'I'.