Dear N:

You wrote:
> Which of these sentences is correct:
>
> 1-I talked to a man at John’s party who works for the phone company.

This is normal spoken English. The modifier is misplaced but understandable. It would be more precise to say “I talked to a man who works for the phone company at John’s party.” That would be the way you would want to write it.

> 2-I talked to a man at John’s party working for the phone company.

The modifier is misplaced here also but is completely ambiguous. Does “working for the phone company” modify “I” or “man”? (With the first one since the verb is “works,” we know you cannot be talking about “I.”)
>
> 3-I took a girl to the cinema yesterday who looks like Mary.

This is occasionally spoken by English speakers but it is awkward at best. Again, the modifier is misplaced. It sounds like the cinema looks like Mary–say instead, “I took a girl who looks like Mary…”

> 4-I took John’s sister to the cinema yesterday, who looks like Mary.

The comma does not help. The modifier is still misplaced. If you were to place it properly, you would use a comma in this case because the modifier is probably nonrestrictive. “I took John’s sister, who looks like Mary, to the cinema…”

I hope this helps. Be sure to check the Grammar Slammer material on Misplaced Modifiers.