一、从结构上看限制性定语从句通常紧接先行词,主从句之间一般没有逗号;非限制性定语从句与先行词之间常有逗号隔开。如:My brother who is18years old is a collegestudent.我18岁的那个弟弟是个大学生。My brother,who is18years old,is a collegestudent.我弟弟是个大学生,? First, from the structural point of view, the restrictive attributive clause is usually followed by an antecedent. There is usually no comma between the principal and subordinate clauses; the non-restrictive attributive clause is often separated from the antecedent by a comma. Such as: My brother who is 18years old is a collegestudent. My 18-year-old brother is a college student. My brother, who is18years old, is a college student. My brother is a university student.