ELLISION OF THAT

Bridge Verbs and “That”

In both of the examples in which that is optional, the relative pronoun is the object of the embedded clause.

Long books [that] religious people like tend to be Bibles. [Religious people like long books.]

Water tanks [that] fish need are spacious. [Fish need water tanks.]

This is also allowed when the relative pronoun is the object of a preposition or another oblique argument of the embedded clause:

This is the boat I escaped in. [I escaped in this boat.]

In your other examples, the relative pronoun is the subject of the embedded clause:

Those that are rotten must be thrown away. [Those are rotten.]

Cars that break down endanger pedestrians. [Cars break down.]

English only allows you to omit that when it has been moved from a non-subject position in the embedded clause, and when it's followed by the subject of the embedded clause. I suspect that the reason for this is the ease of comprehension on the part of the listener. A sentence like Cars break down endanger pedestrians, if it were grammatical, would be extremely hard to parse. (dissect, analyze, decipher, explain, figure out)

Relative clause "that" can be omitted before the subject of a relative clause. (It doesn't have to represent the object of the relative clause, though it may -- "This is the boat (that) I escaped in".)

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 examples of non-bridge verbs: confirm, acknowledge, determine, assert, suggest, demand, indicate, find, etc.  Note that we use many of these verbs in scientific writing (especially determine, assert, suggest, indicate, and find--often the past tense of these, as in "the researchers found that...").  Let's look at some sample sentences.


"That" after nouns: 

As with verbs, there are some nouns that don't require a "that" and some that will sound awkward or confusing without one.  In general, it's a good idea to trust your ears (yet another good reason to read your writing out loud).  Beyond that, here are a couple of nouns that don't always need to be followed by "that:" 

Feeling:  I got the feeling I wasn't alone in the house.
Possibility:  There's a possibility we may have to leave early tonight. 

To clarify, these are instances where the use of "that" would be optional.  I'm not saying it would be wrong to use it.  "There's a possibility that we may have to leave early tonight" is perfectly grammatical. 

Here are a few examples of sentences that would sound awkward without "that."  The nouns are in boldface. Read these aloud as-is, and then read them again, omitting the "that" to see the difference: 

Somehow, he got the idea that I was gossiping about him behind his back.
The fact that I graduated high school a year early doesn't mean that I'm a child prodigy.
Despite rumors that she misused city funds during her four-year term as mayor, Jane Doe seems determined to stay active in local politics.
Following allegations that they tampered with evidence in several high-profile homicide cases, the Everytown County forensics lab is facing investigation by the state attorney general.

"That" after adjectives: 

Again, there are adjectives that need "that" and adjectives that don't.  A big part of determining which is which is simply reading the sentence aloud and trusting your ears.  A good rule of thumb, though, is that common adjectives with broad meanings ("happy," "sad," "glad," etc.) can usually take "that" or leave it--"that" in these cases is optional, as in the sentences "I'm so happy I met you" or "I'm so sad you can't make it Friday night."

 Adjectives with more finely tuned, specific meanings ("ecstatic," "incredulous," "furious," etc.) usually sound better when followed by "that," as in the sentences "I'm ecstatic that I got that job" and "He was furious that you refused to come to his nephew's bris." 

When in doubt... 

Generally, if you're in doubt about whether to use the word "that," put it in.  It's usually safer to include it than to omit it, and as I pointed out earlier, there are very few instances where it would be WRONG to use it.