Here are the sentences completed in the past continuous tense:

1. The scientists (1. discover) **were discovering** a new species in the rainforest.
2. She (2. not/finish) **was not finishing** reading the entire book, even though she started it weeks ago.
3. They (3. work) **were working** on this project for several months, and it’s still not complete.
4. I (4. never/try) **was never trying** such an unusual dish before.
5. The students (5. already/submit) **were already submitting** their essays to the teacher.