Complete sentence has both subject and predicate. Subject is the person who is performing the action and predicate usually contains the verb and the rest part of the sentence. It has complete meaning or makes sense. Incomplete sentence would have either of the two missing.
The given phrase "If I had intended to buy the car" is an incomplete sentence. There should be a second part or predicate of the sentence as well. For example: what would have happened or what would be done, "If I had intended to buy the car". It only mentions the subject part of the sentence.
