personally, I believe it's a big problem.

我个人认为这是个大问题。

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1.In my opinion,somking is bad for your health.依我看，吸烟对你的健康不利。

2. I think she is mistaken.我认为她错了。

3.To be quite frank, nobody's interested in it.坦白地说，没有人对此感兴趣。

4.I'd like to point out that we're in great need of water.我想指出，我们急需用水。