Those were the days when we used to play in the park.
Here the word 'Those' is a -

• 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.