"I know our opinion."
Translation:Ego nostram sententiam scio.
is it always m/um when i am the main subject?
No, no link with the subject, it's a matter of grammatical role in the sentence. Here "our opinion" is a direct object of the verb "to know".
We know what? Our opinion!
So, it requires an accusative (= direct object's case), and not a nominative (= subject's case).
You need to learn cases, and to talk with cases vocabulary, not "main subject" as it would be confusing for you.
The "I" being subject has no link.
Edit: I created a topic specially for you, as I saw you were struggling with cases: