Personlly,I believe it's a big problem. 我認為這是個大問題.
In my opinion,smoking is bad for your health. 依我看，吸煙會危害你的健康.
I think she is mistaken. 我認為她錯了.
To be quite frank,nobody's interested in it. 坦白的說，沒人對此感興趣.
I'd like to point out that we'are in great need of water. 我想指出，我們急需用水.