1. They are in the box. - They're in the box
2. They are happy. - They're happy.
3. They are my friеnds - They're my friеnds.
4. They are doctors - They're doctors.
5. They are good pupils - They're good pupils.
В этом случае после слова they, ставим запятую(вверху), и от слова are берём re, получилось they're.