Hi Sue, Both of your sentences are correct. It depends are who is eating the cookies. If there are two or more people eating cookies, then "they were eating cookies. If it is was a friend you saw, it would be "He/She was eating cookies when I came home from school."

They are both correct. The pronoun could be they, she or he. Some transgender people prefer they to he or she as well.

Both are correct.

The second one is correct. 'Someone' is singular. I saw some people, they were eating cookies. 'Plural'.