Замени существительные на местоимения: My friends are playing outside. → ___ are playing outside. The cat is sleeping on the sofa. → ___ is sleeping on the sofa. My sister and I like to read books. → ___ like to read books.

Выбираем личные местоимения по числу и лицу:

1. My friends — это несколько людей: They are playing outside.
2. The cat — это одно животное: It is sleeping on the sofa.
3. My sister and I — это «мы»: We like to read books.

Ответ: They are playing outside. It is sleeping on the sofa. We like to read books.