Questions & Answers about No quiero ese libro; prefiero el otro.
Spanish often drops subject pronouns because the verb ending already shows who the subject is.
- quiero = I want
- prefiero = I prefer
So Yo no quiero ese libro; yo prefiero el otro is possible, but it sounds more emphatic. Without yo, the sentence is more natural in most contexts.
You would add yo if you want contrast or emphasis, for example:
- Yo no quiero ese libro; yo prefiero el otro.
That can sound like I don’t want that book; I prefer the other one with extra stress on I.