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If you didn't want to buy that shirt you shouldn't have bought it ('didn't' - it's a past simple not a second conditional, watch the video by the link above)
Question:
When can I use a past perfect in the condition part in this sentence (If you hadn't wanted to buy that shirt you shouldn't have bought it)
The wanting or not wanting the shirt happened at about the same time as the buying, so they should have the same tense. You're referring to a linear narrative in the past:
I didn't want the shirt, but I bought it anyway.
The past perfect is used when you need to break away from your linear narrative and mention something that happened further in the past. For example:
I wanted the shirt, but I had already spent $50 on new shoes the day before, so I decided not to buy it.
It doesn't have to be a long time in the past; it just has to be a break from the narrative. For example:
Yesterday I went shopping. I saw a red shirt I liked, but it was twice as expensive as a blue one I had just passed by that was just as good, so I decided to get the blue one instead.
If you did not want to or if you had not wanted to buy that shirt, you should not have bought it.
I think both the past perfect tense and the past tense have the same meaning in the sentence.
Here meaning is more important than grammatical correctness.( Here he bought the shirt anyway)
Though according to the rules the past perfect is the preferred form, there are many violations of this rules by the native speakers.The person teaching the class is a native speaker. So we can not question the correctness of the sentence.
I disagree with the other answers. In the context of an if clause, the so-called past perfect has nothing to do with a past-in-the-past viewpoint, but almost always signifies an irrealis (counter-factual) condition.
If you hadn't wanted to buy the shirt implies that you did want to buy it, and I would expect it to be followed by a consequent such as you wouldn't have gone to the shop.
If you didn't want to buy the shirt could have an irrealis reading, but could also have a realis one, as in If you didn't want to buy the shirt, you shouldn't have gone to the shop.
Past Perfect, as I was tought, is a structure used to emphasize the fact that one action had happened in the past before another action happened. And this is pretty much what is happening in your sentence. The order of actions were:
First - you hadn't wanted to buy this shirt
Second - you bought it anyway
That's why it is normal and natural for you to use the 3rd conditional form.
But then, why the mixed conditional has to be be used? here's why... The 2nd-3rd conditional is more general. The first part of the sentence in mixed conditional has to be constant and that's what is happening in this sentence.
The person didn't want to buy this shirt and they probably still regret doing so and that's why the action (not wanting to buy the shirt) is expressed in the 2nd conditional.
Answering your question: when could you use a pure 3rd conditional?
Well, in the moment when it is emphasized that the person doesn't regret buying this shirt after all.
