Model: The officer, who stood at the entrance, was the first to notice the fire. — The officer who stood at the entrance was the first to notice the fire.
In the first sentence the attributive clause is non-restrictive. It gives additional information about the antecedent (the officer) and suggests that there was only one officer: the clause does not serve to single him out of a group. In the second sentence the clause may be either non-restrictive (in which case the same explanation stands good), or, rather, restrictive; in the latter case the clause serves to single the antecedent out, and suggests that there were several officers and the fire was noticed by the one who stood at the entrance.
1. The sonnet form, which was first used by the Portuguese poet Luiz de Ñàòî¸ïý in the 16th century, had been popular for four hundred years (Lee). 2. Even the dog got up and stood by the door, which I had left open (Bullett). 3. The policeman, who had been staring impassively at the floor, looked up (G. Greene). 4, He put the case, which was very heavy, on the seat beside him (Innes). 5. Tom's brother, who lives in Birmingham, is an engineer (Hornby) 6. The Browns, who lived in the adjoining house, had been dining with the Joneses (Leacock).