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.