"That is what I want." The word "that" is -

• "That is what I want." the word "that" is a pronoun (demonstrative pronoun).
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•'That' - pronoun হবার কারণ:
- 'That' এর পরে verb অথবা of থাকলে 'That' demonstrative pronoun হবে। 
- নিয়মানুযায়ী, That is what I want - বাক্যে that হচ্ছে pronoun কারণ, that এখানে একক ভাবে ব্যবহৃত হয়েছে এবং এরপরে কোনো noun নেই।

- Demonstrative pronouns are used to point to specific things or people. In this sentence, "that" is referring to something specific, indicating the thing or idea being discussed or mentioned. It functions as a pronoun replacing a noun or noun phrase, and it helps to clarify or specify the desired object or concept.
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• Demonstrative Pronoun:
- ব্যক্তি বা বস্তুকে যদি এটি, ওটি, এগুলি, ওগুলি ইত্যাদি শব্দ দ্বারা নির্দেশ করা হয় তাহলে এই শব্দগুলোকে বলে Demonstrative pronoun.
- যেমন: This, that, these, those. 

Types of Pronouns: 
1. Personal Pronoun: I, you, they, he, she, etc. 
2. Interrogative Pronoun: Who? What? which? etc. 
3. Distributive Pronoun: Each, either, neither, etc. 
4. Demonstrative Pronoun: This, such, that, Those, etc. 
5. Relative Pronoun: Who, what, which, that, etc. 
6. Reciprocal Pronoun: Each other, one another, etc. 
7. Reflexive Pronoun: Myself, yourself, herself, etc. 
8. Indefinite Pronoun: One, any, some, etc.

Source: A Passage to the English Language by S. M. Zakir Hussain.