In what situation would somebody use the clause the car which is yellow? For example: the car which is yellow is mine, the car which is blue is yours and the car which is red is John’s. If I say the car which is yellow, am I giving you more information about a particular car we were already talking about by telling you its colour – or am I helping you to identify the car by telling you that it is the yellow one rather than the red one or the blue one?
We are looking here at two different kinds of relative clause, which we can call non-restrictive and restrictive relative clauses. Non-restrictive relative clauses add extra information; restrictive relative identify something for us by picking it out of a group of similar things.
The relative clause gives you information to help you identify the correct car and to narrow down our choices from a set of options.
How about this example?
The car, which is yellow, is on the left and the bicycle, which is blue, is on the right.
Here, the relative clause adds some extra information about the car, but it was already clear which car we were talking about.
We call this a non-restrictive relative clause.