I have a dataframe where every row is a word or punctuation. I want to iterate through the dataframe and whenever a row contains punctuation, I want to combine it with the previous row. 

For example, I want to convert:

```html
   word 
0  hello      
1  ,
2  how
3  are
4  you
5  ?  
```

Into:

```html
   word 
0  hello,      
2  how
3  are
4  you?
```

Thanks.