When you retweet with the standard Twitter button, you are prompted to add a comment. This is a newish feature at Twitter (summer of 2015), and it is really fraught with peril: I do NOT recommend it. Details here.
What I do STRONGLY RECOMMEND is the Classic Retweet Extension which is available for both Chrome and Firefox browsers. This extension adds an extra "retweet" option to your Twitter display which allows you to edit your retweet before you publish it. It shows up just to the left of the standard Twitter retweet icon:
If you make more extensive edits to the retweet, you can always change the RT at the start of the tweet to MT, "modified tweet."
To find out more, here is additional information about the Classic Retweet Extension. Highly recommended!