Every now and then, I'll hear writers talk about searching their manuscripts for "that" and cutting the word from the text. You don't need it, they say. It's just clutter, they say. And so they type those for little letters into a find bar and cut away.
This worries me a little because the word does serve a purpose, and mere mindless cutting can create problems in the text. This is not an idle worry. I've seen these problems in manuscripts. Let's think about the ways we use that, and let's see if we can figure out when the usage is good. We'll start today with that as a pronoun.
That is her car in the third space.
In this sentence, that is being used as a pronoun which substitutes for the noun car. The clause That is together form the main clause of the sentence, with the direct object her car and the prepositional phrase in the third space completing the predicate. It's a grammatically correct sentence. To eliminate that, you would either need to use a different pronoun --
It is her car in the third space.
Or you would need to revise the sentence to eliminate the pronoun altogether --
Her car is in the third space.
Either of these sentences is grammatically correct, but the differences between them are vast, to my eye. The first sentence, the one with that, to me appears to be emphasizing the specific car as distinguished from any other cars -- her car is not the one in the first space or second space. Her car is the one in the third space. That car, not those other cars.
The second sentence, with it, seems to me to be asserting ownership. It is her car, not his car, not your car, and not my car. The difference between the first and second sentences is subtle, though, because both are using structures of emphasis. This distinguishes them from the third sentence, which is stripped of any such emphasis. The third sentence is a mere statement of fact, providing the location of the car and identifying it as belonging to her. Neither of these facts are emphasized.
In some cases, you will want the emphasis, and in some cases, you will not. Much depends on context. So when you're looking at a pronoun usage of that, don't just cut it without evaluating whether it is being used as a means of emphasis. Most of us will make this decision on autopilot and get it right, but with all the that-cutting advice floating around the writersphere, make sure your autopilot doesn't get compromised!