Generally, if there is a specific line or two from someone's post you would like to quote you can either:
1. Highlight it, copy it, and then paste what you've copied into your own box. Type
at the end of your selection.
at the beginning and
2. Hit the QUOTE button in the lower right hand corner, then edit out all but your selection.
It's much better than inadvertently everyone quoting each other's complete posts.