When You Should Omit the Word “That”
Leaving “that” out sounds best with the most common verbs of speech or thought, such as “say,” “think,” “know,” “claim,” “hear,” or “believe.” It saves a word, and it’s how people talk, too. Linguists call these verbs “bridge verbs.”
Non-bridge verbs tend to be verbs that carry extra meaning beyond simply the idea of saying or thinking something, and they don’t sound as good when you omit the word “that.” For example, “whisper” is a non-bridge verb and doesn’t mean just to say something; it means to say it in a particular way. It sounds odd to say, “He whispered he wanted another root beer” instead of “He whispered that he wanted another root beer.” Not crashingly bad, but just a little off.
More: Grammar Girl : Omitting ‘That’ :: Quick and Dirty Tips TM