Personlly,I believe it's a big problem. 我认为这是个大问题.
In my opinion,smoking is bad for your health. 依我看，吸烟会危害你的健康.
I think she is mistaken. 我认为她错了.
To be quite frank,nobody's interested in it. 坦白的说，没人对此感兴趣.
I'd like to point out that we'are in great need of water. 我想指出，我们急需用水.