Elementary

1. I (really) think that …

2. I believe (that) …

3. I’m sure that …

4. In my opinion / My opinion is …

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Pre-intermediate

1. I agree with …

2. I feel that …

3. I guess/imagine …

4. I have no doubt that / I’m certain that …

5. I strongly believe that …

6. I’ve never really thought about this before, but …

7. My personal opinion is that / Personally, my opinion is that …

8. To be honest / In my honest opinion, …