"This is a good book."
Translation:זה ספר טוב.
Because "הספר טוב" means "the book is good", and the book is the subject of the sentence. The sentence "זה ספר טוב" means "this is a good book", so "this" is the subject. (My grammar explanations are for the English translations--I don't know enough about Hebrew grammar to say whether the same terms are used.)