this and that.This and that are both demonstrative pronouns that refer to a thought expressed earlier: The letter was unopened; that (or this) in itself casts doubt on the inspector’s theory. That is sometimes viewed as the better choice in referring to what has gone before (as in the preceding example). When what is referred to has not yet been mentioned, only this is used: This (not that) is what bothers me: we have no time to consider late applications.
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this as informal substitute for a / an.This is often used in speech and informal writing as an emphatic substitute for the indefinite article to refer to a specific thing or person: You should talk to this friend of mine at the Department of Motor Vehicles. I have this terrible feeling that I forgot to turn off the water. But it’s best to avoid this substitution in formal writing except when you want to create a conversational tone.
