So I'm still really confused on the difference between them and how to use them in sentence. When should I use 이 or 이거 in a sentence or one instead of the other? How is one a pronoun and one a modifier? I'm sorry if these all seem like really basic questions but even though I'm a native English speaker, I've never been able to understand or memorize English grammar. If someone can explain this in depth but also simply then that would be great!
Edit: Would I use 이 to say "this (noun)" and then say something about the noun like "this book (이 책) is red?" Or would I use 이거 in this instance?