This is the complete sentence using the correct form of ser:
- Los dinosaurios eran animales muy grandes.
The verb eran is conjugated in the imperfect tense and eran is the only one that is right for "los dinosaurios", which is the third person of the plural form.
Imperfect tense
The imperfect tense is used to talk about past habitual situations or to describe what someone was doing but then started doing something else.
Check more information about the imperfect tense here https://brainly.com/question/1131180?referrer=searchResults
