CORRECTIONS:
1. I hide behind the computer.   correct!
2. My friend hides beside the swing.
3. My teacher hides under the table.

Explanation: You need to follow the rule that regards subject-verb agreement. Wherein if the noun is singular the verb should be in it's singular form.
Ex: She eats dinner every eight o'clock. [Singular]
      They eat dinner every nine o'clock. [Plural]

Comment:
Thank you for sending me your homework, Elsa! I had a great time in our class. Please take note of my corrections and see you again next meeting!
-Teacher Joanna