Rules 11 and 12
11. Unnecessary Shift in Pronoun
We went to the baseball seminar at the high school, and we learned how to throw a curveball.
When you use a pronoun, you need to make sure to keep the same pronoun though out the sentence.
12. Sentence Fragments
My mother went to the store to buy some groceries.
When you write a sentence, you need to make sure that it has a subject and a predicate.