Can demonstrative pronouns be subjects

Yes, the demonstrative pronouns can function as a subjector an object in a sentence.

The demonstrative pronouns are: this, that, these, those.

Examples:

This is a good movie. (subject of the sentence)

What a good day that was. (subject of the relative clause)

Mom made this from scrap material. (object of the verb 'made')

I'll need more time for this. (object of the preposition 'for')