In response to your question. Yes I do believe "That was parked under the tree" can stand alone.
You're right, but in the sentence in your other post, "that" is used as a subordinate conjunction, not a pronoun. Therefore, in that sentence, "that was parked under the tree" is a dependent clause.
To determine if the sentence "That was parked under the tree" can stand alone, we need to understand the concept of a complete sentence. A complete sentence, also known as an independent clause, is a group of words that expresses a complete thought and contains a subject and a predicate.
In the sentence "That was parked under the tree," the subject is "that" and the predicate is "was parked under the tree." It contains both a subject and a verb, and it expresses a complete thought.
Therefore, "That was parked under the tree" can indeed stand alone as a complete sentence. It provides a clear message and can be considered a self-contained thought.