Subject vs Object Relative Pronouns

Look at the word that comes directly after the relative pronoun. If a verb follows, the pronoun is the subject: "The man who called me" — "who" is followed by the verb "called," so "who" is the subject. If a noun or personal pronoun follows, the relative pronoun is the object: "The man who I called" — "who" is followed by "I" (another subject), so "who" is the object. You can test by removing the pronoun: "The man I called" works, but "The man called me" changes the meaning. This test works with "who," "which," and "that."

You can leave out the relative pronoun when it is the object of the clause. "The book (which) I read" — "which" is the object because "I" is the subject doing the reading. This is very common in spoken English and is not considered informal or incorrect. However, you can never leave out the subject pronoun: "The book which is on the table" — "which" is the subject, so removing it gives "The book is on the table," which changes the meaning completely. You also cannot omit pronouns in non-defining clauses or after prepositions in formal writing.

No, removing an object pronoun does not change the meaning at all. "The cake which she made" and "The cake she made" mean exactly the same thing. The only difference is style — the shorter version sounds more natural in everyday spoken English, while keeping the pronoun can feel slightly more formal. In writing, both are equally acceptable. The important thing is that you can only remove object pronouns, not subject pronouns. Removing a subject pronoun would break the sentence grammar: "The cake won the prize" (without "which") is a different sentence from "The cake which won the prize."