Answer: 5. me encierro
6. te sientas
7. se pierde
Explanation: encerrarse is irregular, the 'e' in cerrar turns to 'ie'
sentarse is irregular, the 'e' in sentar turns to 'ie'
perderse is is irregular the 'e' in perder turns to 'ie'
The reflexives are just based on who is doing the action.