Here's why:
* Pronouns stand in for nouns. They replace a noun or noun phrase already mentioned.
* Adjectives describe nouns. They provide more information about a noun.
Let's look at some examples:
Demonstrative Pronoun:
* "This is my favorite book." (This replaces the book, so it's a pronoun)
* "I like that one." (That replaces the one, so it's a pronoun)
Demonstrative Adjective:
* "I like this book." (This describes the book, so it's an adjective)
* "Can you hand me that pen?" (That describes the pen, so it's an adjective)
Key difference:
The demonstrative pronoun stands alone, replacing a noun. The demonstrative adjective is always attached to a noun to describe it.