There are many uses. Here are some: 1. "I think that you are a good student." "That" introduces "you are a good student." The word is not absolutely necessary in that kind of sentence. 2. "This is the book that I like." "That" is absolutely necessary. It refers to "the book." (We would sound like children if we said: This is the book. I like the book.) 3. Tom: What is her name? Joe: What girl are you talking about? Tom: That girl over there by the tree. Joe: Oh, that girl! Her name is Mona.

That = นั้น That car = รถนั้นคัน

1. (used to indicate a person or thing as pointed out or present, mentioned before, supposed to be understood, or by way of emphasis): That is her mother. 2. (used to indicate one of two or more persons or things already mentioned, referring to the one more remote in place, time, or thought; opposed to this): This is my sister and that's my cousin. 3. (used to indicate one of two or more persons or things already mentioned, implying a contrast or contradistinction; opposed to this): This suit fits better than that. 4. (used as the subject or object of a relative clause, esp. one defining or restricting the antecedent, sometimes replaceable by who, whom, or which): the horse that he bought. 5. (used as the object of a preposition, the preposition standing at the end of a relative clause): the farm that I spoke of. ADJECTIVE 6. (used to indicate a person, place, thing, or degree as indicated, mentioned before, present, or as well-known or characteristic): That woman is her mother. 7. (used to indicate the more remote in time, place, or thought of two persons or things already mentioned; opposed to this): This room is his and that one is mine. 8. (used to imply mere contradistinction; opposed to this): not this house, but that one. ADVERB 9. (used with adjectives and adverbs of quantity or extent) to the extent or degree indicated: Don't take that much. 10. to a great extent or degree: It's not that important. 11. Dial. (used to modify an adjective or another adverb) to such an extent: He was that weak he could hardly stand. CONJUNCTION 12. (used to introduce a subordinate clause as the subject or object of the principal verb or as the necessary complement to a statement made, or a clause expressing cause or reason, purpose or aim, result or consequence, etc.): I'm sure that you'll like it. That he will come is certain. that