1)Fornunately
2)unfortunatoly
3)You,see
4) Frankly speaking
5) To make a long story short
6) Besides

1) Fortunately, I know how to do it.

2) Unfortunately, we have no time to do it.

3) You see, we can't go there now.  

4) Frankly speaking, I don't think we can do it.

5) To make a long story short, he didn't know the answer.

6) Besides, we have already done it.