I can say what I think using sentence starters such as "I think..." and "I believe..."
(E.g. I think it was unfair that the king owned all the land.)

I can give a reason for my opinion using "because".
(E.g. I believe it was fair because the king protected the land.)

I can listen to others' opinions and say whether I agree or disagree, giving a reason.
(E.g. I disagree because... / I agree because...)