Here's why:
* "Their" is a possessive pronoun. It shows ownership or belonging.
* Subjects are the things or people performing the action. They need to be nouns or noun phrases.
Example:
* Incorrect: Their went to the park.
* Correct: They went to the park. (They is the subject)
You could use "their" in a sentence with a noun as the subject:
* Correct: Their dog is very friendly. (Dog is the subject)