So replying directly to other users is most easily done by pressing the quote button. As you can see above, I have done that to your previous post and you probably got a neat little notification for it.
If you want to quote a specific portion of another user's post, you use your mouse or touchpad to highlight the part that you want to quote. You will then get a little prompt to quote that selection. Click it and it should let you quote the highlighted text.
I see you already know about tagging. There's no real defined etiquette to it per se and users generally apply its use to whatever they find comfortable. If I were to offer a recommendation, generally I'd say it's better do it when you want to call a user to a thread -- usually in status posts, blogs, or silly threads in forum lounge/forum games -- or you're having a discussion with another user and your response doesn't relate to something they've directly said.
Hope this has been helpful.