Answer:
1. mettaient
2. étudiait
3. ramassaient
4. écoutais
5. finissions
6. nageait
Explanation:
You would have to look at the L'imparfait conjugation chart.
The ending that will be used for the verb in the parenthesis depends on the subject pronoun, that is being used in the sentence.
For example, in number five the word "nous" means "we".
Since the subject pronoun that is used is nous (which means we) we will used the imparfait ending "-ions".
For the (IR Verb finir) the stem for the imparfait is finiss- and then you would add the appropriate ending for nous which is (-ions).
If you need further explanations, please let me know and I would be more than happy to reply back.