In Portuguese an opinion rarely arrives bare. It comes with an opener, and the opener tells your listener how firmly you hold the view. From softest to firmest:

Opener	English
Acho que…	I think…
Eu diria que…	I'd say…
Na minha opinião,…	In my opinion,…
Pra mim,…	For me,… / To me,…
Do meu ponto de vista,…	From my point of view,…
Sinceramente,… / Pra ser sincero,…	Honestly,… / To be honest,…
Tenho certeza que…	I'm sure…
Sem dúvida,…	Without a doubt,…

• Acho que is the workhorse, and lighter than English I think: Brazilians use it to soften almost anything, even plain facts — Acho que a padaria fecha às oito.
• Eu diria que uses the conditional, which softens exactly as English I'd say does.
• Pra is the spoken form of para. In writing: para mim.
• Sincero agrees with the speaker: Marcos says pra ser sincero, Marina pra ser sincera.
• Grammar books write tenho certeza de que; in conversation the de usually disappears.