Grammatically correct: a. The film was so good, I could hardly believe it. b. The film was so good, I couldn't believe it. c. The film was so good, I couldn't hardly believe it d. The film was so good, I could believe it.
Grammatically correct: a. The film was so good, I could hardly believe it. b. The film was so good, I couldn't believe it. c. The film was so good, I couldn't hardly believe it d. The film was so good, I could believe it.
Explanation
The film was so good, I couldn't believe it.
Main clause: The film was so good → Describes the film.
Result clause: I couldn't believe it → Shows the speaker's strong reaction.