Answer:
1. Los turistas ya habían perdido sus pasaportes cuando llegaron al hotel.
The tourists had already lost their passports when they arrived at the hotel.
This sentence is an example of the Past Perfect tense.
2. Paula decidió alojarse en un albergue, pero aún no sabe en cuál.
Paula decided to stay in a hostel, but she still doesn't know which.
This sentence is in the Preterite tense.
3. Mateo había reservado el hotel desde hacía cuatro meses.
Mateo had booked the hotel four months ago.
This sentence is also in the Past Perfect tense.
4. Se hace tarde y nosotros aún no terminamos de hacer las maletas.
It's getting late and we haven't finished packing yet.
Another example of the Preterite tense.
5. Hoy ellos recorrieron el centro histórico de la ciudad.
Today they toured the historic center of the city.
This sentence is also a the Preterite one.
6. ¡Nunca antes había estado en un crucero! ¡Lo disfruté mucho!
I had never been on a cruise before! I enjoyed it a lot!
This last sentence is in the Past Perfect form.