"How about That?"
by Trina Pink
"That" is used to tie critical information into a sentence. For instance, "The house that collapsed had been flooded the previous spring." We need the phrase "that collapsed" or we wouldn't know which house we're discussing. In this case, the word "that" is needed to clue us in that the following information is necessary. On the other hand, if you remove the word "that," the sentence no longer makes sense. You can also use "that" as a pronoun to distinguish an object from "this," another object (often one that is closer). For example, "I want that cookie, not this one."
However, we often add "that" where it serves no purpose at all, especially in casual speaking and colloquial writing. For example, "I thought that I needed to go to the bed." "She said that she was depressed." In these sentences, if you remove the word "that" and read the sentence, it still makes sense and means exactly the same thing. So you know the word is extraneous here and should be removed. (Unfortunately, since we hear and see it used like this so often, we cannot count on our "ear" for picking it up; we have to actually check.)
Another way to see whether you need to keep the word "that" or not is to replace it with the word "which" and read the sentence. If it still makes sense, you need to keep either "that" or "which." For instance, the sentence above, "The house which collapsed had flooded..." still makes sense using "which," so we know to keep one of the words. "She said which she was depressed" makes NO sense at all--lose that "that!"
How do you know when to use "that" and when to use "which?" "That" is used for information critical for understanding; "which" is used for interesting details that could be removed without changing the meaning. For instance, "I was dumped by my boyfriend, which didn't exactly break my heart." Or "I decided to buy the blue dress, which matched my eyes" (the point being, we know which dress it is...the BLUE one...so we don't need the clause that follows "which.").