difference between the and a in this context with relative clause

Question

What is the difference between those sentences?

He is the person I needed for the job.

He is a person I needed for the job.

And...

This is not a place I wanted to go.

This is not the place I wanted to go.

I think there is a difference in nuance, but I'm not sure about the specifics.

This is one of the explanations I found online, but is it the same as my examples?

He bought the house with a big backyard. This combination tells the listener which specific house he bought.

He bought a house with a big backyard. This combination tells the listener what kind of house he bought, but not the specific house he bought.

I assumed that one with "the" implies that the noun it modifies is one, one and only. However, when "a" gets in the way, the noun it modifies becomes one of many, or at least implicitly suggested that there can be more than one.

Is my guess correct?

Edit: Here is the thread I found. http://www.grammaring.com/defining-relative-clause

An identifying clause defines who or what we are talking about and refers to a noun which is preceded by the definite article the:

This is the book I told you about last week. Remember? Do you know the guy who is talking to Will over there?

A classifying clause describes what kind of person(s) or thing(s) we are talking about. It refers to a singular uncountable or plural countable noun without an article or a singular countable noun preceded by the indefinite article a or an:

I wrote my essay on a photo which was taken by Robert Capa. Sometimes, I like listening to music that makes me sad. I don't know any people who speak Esperanto.

So is it fine to say "this is a book I told you about last week" if I am trying to classify it instead of identifying it?

Answer 1

The first thing to say is that this is subtle, and it's sometimes hard for native speakers to put into words just why they would choose "the" vs "a" or vice versa, even though it's clear to them which one they would use.

For "the person I needed for the job" vs "a person I needed", your intuition is right that the first sentence implies that you needed one particular person, and you found him. Even though that's hardly ever literally true, it's a nice way to emphasize how ideal someone is, and so it's by far the more common thing you would say. (But the second sentence is a perfectly good way of saying "I needed this person for this job.")

A similar thing is true with the second pair of sentences. When you say "This is not the place I wanted to go," you imply that there was a specific place you wanted to go, and this isn't it.

If you say "this is a book I told you about yesterday," it might suggest that it was one of several books, or it might mean that you don't think I remember it. "The" essentially points to a specific item, so if I've forgotten the conversation, it's less natural to point to it.