I was reading about relatives clauses into a book, I read book that when the relatives pronouns are subjects of the verbs I cannot leave them out, and whe the relative pronoun are objets of the verb I can leave them out.
I can said:
The men who lives next door
but I cannot said:
The men lives next door (without who)
Both sentences are correct:
The dress that Ann bought doesnīt fit her very well
The dress Ann bought doesnīt fit her very well (without that)
someone could help me... I donīt understand when a relative pronoun is the object or the subject of the verb. What is the meaning of these?
If you look at the verb the second example (bought), then Ann bought the dress, so she is the subject (who did the action) and the dress is the object (what she bought. The first, we only have the men and they are the subject of the verb (they live next door)
In addition, the relative clause "who live next door" functions as an adjective. It tell us more about the noun "men":
 The men who live next door like pizza. (Relative Clause)
The structure of the underlined clause is as follows: it has a verb "live" and a subject "who", and the verb "live" agrees in person and number, not with "who", but with the noun that "who" modifies.
EX: The men who live . . . . (plural subject and verb)
EX: The man who lives . . . . (singular subject and verb)
Take out the relative clause "who live next door" and we get the sentence's main or true subject ("the men") and its main or true verb ("like):
EX: The men like pizza.
EX: The menwho live next doorlike pizza.
Now, if we leave out "who", and keep the rest "live next door", what happens is "the men" becomes the subject of "live", leaving "like" without a subject:
EX: *The men live next door like pizza. (ungrammatical)
That part, "The men live next door" is grammatical, but the rest of the sentence isn't. You see, "like" doesn't have a subject, and in English subjects are important. To make the sentence grammatical, we'd have to add a subject:
EX: The men live next door and they like pizza.
In short, the relative clause needs "who". It functions as a subject. Below, in , the relative clause doesn't need "that". It doesn't function as a subject. "Ann" is the subject of "bought":
The dress thatAnn bought doesnīt fit her very well.
"that" isn't necessary, so you can omit it:
The dress Ann bought doesnīt fit her very well.